In how many ways can Rs. 18.75 be paid in exactly 85 coins consisting of 50-paisa, 25-paisa and 10-paisa coins?
8
Suppose we take x = 50-paisa coins, y = 25-paisa coins and z =10-paisa coins.
We have x + y + z = 85 and 50x + 25y + 10z = 1875, because x, y and z are all integers, z should be a multiple of 5.
Possible values are
x y z2 63 205 55 258 47 30 .... ....23 7 55
So there are eight solutions.
You should notice here that an increase of 5 in z and 3 in x and decrease of 8 in y neither changes the number of coins nor the value.