Q4. If is a multiple of 3, where is a digit, what might be the values of ?
If a number is multiple of 3, the the sum of its digits will be multiple of 3.
Sum of digits of 31z5 = 3 + 1 + z + 5 = 9 + z
9 + z is a multiple of 3 and since z is a single digit
9 + z is any one of 9, 12, 15 or 18
thus value of z is 0, 3, 6 or 9 respectively.
Hence possible value of z is 0,3,6 or 9.