There are two flasks M and N containing 130 ml and 180 ml of two different concentrations of acid. Now 'x' ml is withdrawn from each flasks, and then the amount withdrawn from M is added to N and vice versa. Finally the concentrations found to be equal in both the flasks. Find x?

(130a-ax+bx)/130 = (180b-bx+ax)/180
180*130a - 180ax + 180bx = 180*130b - 130bx + 130ax
=> 310x(b-a) = 180*130*(b-a)
Cancel (b-a) as we know that the concentrations are not equal.
310x = 180 * 130
x = 75.48

Thanks for the solution. But how to solve the equation (130-ax+bx)/130 = (180b-bx+ax)/180?

Let a% and b% be the concentrations of M and N respectively => Initial volumes of acid in the solutions M and N are 130a and 180b respectively.
Final volumes of acid in the solution after transferring x ml from each of the flasks to the other flask are 130a-ax+bx and 180b-bx+ax.
Final concentrations are equal => (130-ax+bx)/130 = (180b-bx+ax)/180
On solving this equation you get the value of x as 75.48 ml.