# Q. Find the squares of the following numbers containing 5 in unit’s place.(iv) 205

Consider a number with unit digit 5, i.e.,    a5

=  = 10a(10a + 5) + 5(10a + 5) = 100a 2 + 50a + 50a + 25

= 100a(a + 1) + 25

= a(a + 1) hundred + 25.

Here a = 20

Hence     =    20(20+1)hundred + 25

=   20(21)hundred + 25 = 42000 + 25

=   42025

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