if n veries inversely as (y^2-1) and is equal to 24,when y=10. then find the value of n, when y=5

n = k/(y^2-1)

So, n(y^2-1) = k, a constant.

You want n such that

n(5^2-1) = 24(10^2-1)
24n = 24*99
n = 99