Solution:Option (a)
The question is asking for the remainder when 21643 is divided by 25. We can use the last two digits technique to get the answer, as the remainder when a number is divided by 25, depends only on the last two digits i.e. 21643=......61(6=2×3)
when 61 is divided by 25, the remainder is 11. Answer is option (a)
For numbers ending in 1, the last two digits of a power expression ab can be determined as
The last digit is always 1
The 2nd last digit = product of tens digit of base * unit digit of the power.