#### Fuel contamination levels at each of 20 petrol pumps P1, P2, …, P20 were recorded as either high, medium, or low. 1. Contamination levels at three pumps among P1 – P5 were recorded as high. 2. P6 was the only pump among P1 – P10 where the contamination level was recorded as low. 3. P7 and P8 were the only two consecutively numbered pumps where the same levels of contamination were recorded. 4. High contamination levels were not recorded at any of the pumps P16 – P20. 5. The number of pumps where high contamination levels were recorded was twice the number of pumps where low contamination levels were recorded.Question: If the contamination level at P11 was recorded as low, then which of the following MUST be true? Option: 1 The contamination level at P12 was recorded as high.     Option: 2 The contamination level at P14 was recorded as medium.   Option: 3 The contamination level at P15 was recorded as medium.   Option: 4 The contamination level at P18 was recorded as low.

Based on the given conditions 1 and 2, we can derive the following results:

 P1 P2 P3 P4 P5 P6 H M H M H L

Additionally, from condition 4, we encounter two possible cases:

 P16 P17 P18 P19 P20 L M L M L M L M L M

According to condition 5, we have two scenarios:

If the total number of low (L) pipes is 3:

• Number of high (H) pipes: 6
• Number of medium (M) pipes: 11

If the number of low (L) pipes is 4:

• Number of high (H) pipes: 8
• Number of medium (M) pipes: 8

For P7 and P8, they can either be HH or MM, resulting in two distinct cases for P1 to P10.

 1 2 3 4 5 6 7 8 9 10 H M H M H L M M H M H M H M H L H H M H

Combining conditions 1 and 2, we find the following potential cases for P1 to P20:

Case 1: H M H M H L H H M H M H M H M L M L M L

• Number of (L) pipes: 4
• Number of (H) pipes: 8
• Number of (M) pipes: 8

Case 2: H M H M H L H H M H L M H M H M L M L M

• Number of (L) pipes: 4
• Number of (H) pipes: 8
• Number of (M) pipes: 8

Case 3: H M H M H L H H M H M L H M H M L M L M

• Number of (L) pipes: 4
• Number of (H) pipes: 8
• Number of (M) pipes: 8

First, we create a table and fill in the information provided with absolute certainty.

 Pumps Contamination Level P1 P2 P3 P4 P5 P6 Low P7 P8 P9 P10 P11 P12 P13 P14 P15 P16 P17 P18 P19 P20

According to statement 3, pumps P7 and P8 are the only consecutive pumps with the same contamination level recorded.

Statement 1 indicates that three pumps among P1 - P5 have high contamination levels. Therefore, pumps 1, 3, and 5 must have a high level of contamination. Additionally, P6 is the only pump among P1 - P10 with a recorded low contamination level. Hence, pumps 2 and 4 have a medium level of contamination.

 Pumps Contamination Level P1 High P2 Medium P3 High P4 Medium P5 HIgh P6 Low P7 P8 P9 P10 P11 P12 P13 P14 P15 P16 P17 P18 P19 P20

It is stated that high contamination levels were not recorded at any of the pumps P16 - P20. Consequently, we can deduce that high contamination was only recorded in the first 15 pumps. This means the maximum number of pumps that can have a high contamination level is '8' (considering consecutive pumps have different contamination levels, except for one case).

Moreover, it is mentioned that the number of pumps with high contamination levels is twice the number of pumps with low contamination levels. Thus, the number of pumps with high contamination is an even number less than or equal to '8'.

If we assume that there are '6' pumps with high contamination levels, there would be only '3' pumps with low contamination levels. This would imply that we need 11 pumps with medium contamination levels (20 - 6 - 3), which exceeds the limit of 10 pumps for a single type of contamination level (due to consecutive pumps having different contamination levels, except for one case).

Therefore, we can conclude that the number of pumps with high contamination levels is 8, the number of pumps with low contamination levels is 4, and the number of pumps with medium contamination levels is 12.

Considering that P7 and P8 are the only consecutive pumps with the same contamination level, if they both recorded a medium contamination level, the maximum number of pumps that can have a high contamination level is 7 (P1, P3, P5, P9, P11, P13, P15). Thus, we can deduce that pumps P7 and P8 recorded a high contamination level, allowing us to determine the contamination levels uniquely up to P10.

 Pumps Contamination Level P1 High P2 Medium P3 High P4 Medium P5 High P6 Low P7 High P8 High P9 Medium P10 High P11 P12 P13 P14 P15 P16 P17 P18 P19 P20

Furthermore, as high contamination levels were not recorded at any of the pumps P16 - P20, we can infer that these 5 pumps recorded low and medium contamination levels. There are two possible cases:

Case 1: When there were 3 pumps with low contamination and 2 pumps with medium contamination recorded in pumps P16 - P20. In this scenario, P16, P18, and P20 would have low contamination levels.

 Pumps Contamination Level P1 High P2 Medium P3 High P4 Medium P5 High P6 Low P7 High P8 High P9 Medium P10 High P11 Medium P12 High P13 Medium P14 High P15 Medium P16 Low P17 Medium P18 Low P19 Medium P20 Low

Case 2: When there were 2 pumps with low contamination and 3 pumps with medium contamination recorded in pumps P16 - P20. In this situation, P16, P18, and P20 would have medium contamination levels.

We can see that in case 2(a) the contamination level at P11 was recorded as low. Let us check all the option one by one.

(Option: 1) The contamination level at P12 was recorded as high. This statement is incorrect as we can see in the table, the contamination level at P12 was recorded as medium.

(Option: 2) The contamination level at P14 was recorded as medium. This statement is correct as we can see in the table, the contamination level at P14 was recorded as Medium.

(Option: 3) The contamination level at P15 was recorded as medium. This statement is incorrect as we can see in the table, the contamination level at P15 was recorded as High.

(Option: 4) The contamination level at P18 was recorded as low. This statement is incorrect as we can see in the table, the contamination level at P18 was recorded as Medium.

Hence, option 2 is the correct answer.