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Let "w" be the increase of the length and the width.
Then our governing equation is
(10+x)*(3+x) = 2*10*3 = 60.
One way is to solve it as a quadratic equation.
I will choose another way.
My task is to find a decomposition of the number 60 into the product of two factors, that differ in 10-3 = 7 units.
As soon as you reformulate the problem in this way, it is a matter of seconds to guess the answer. It is
60 = 12*5.
So, the new dimensions of the pool are 12 meters and 5 meters.
Check. 12*5 = 60 = 2*30; 12-10 = 2 = 5-3.
Solved.