1. The problem statement, all variables and given/known data

There are 2 beakers P and Q. P contains 50 litres of water and Q contains 40 litres of acid. x litres is taken from both the beakers, mixed and replaced. This process is repeated 2 times and after the second time, there is 71/8 litres of acid in P. What is the quantity of x ?

2. Relevant equations

Form the equation in the form of ax^2 + bx + c = 0 where a is not equal to 0 and a,b and c are constants.