When 3 is added to both numerator and the denominator of a certain fraction, its value is decreased by 9/40. However, when 3 is subtracted from both, its value is increased by 9/10. Find the denominator of the original fraction.

Solution:

The fraction is x/y

(x+3)/(y+3) + 9/40 =x/y

(x-3)/(y-3) -9/10 =x/y

So

x=8

y=5

So the denominator of the original fraction is 5