A total of 324 coins of 20 paise and 25 paise make a sum of Rs 71 The number of 25 paise coins is ........

Explanation:

Let the number of 20-paise coins be x. Then, number of 25-paise coins = (324 - x).
Therefore 0.20 x x + 0.25 (324 - x) = 71   20x + 25 (324 - x) = 7100
5x= 1000   x = 200. 
Hence, number of 25-paise coins = (324 - x) = 124.