To simplify the expression (25x^2 - 20xy + 4y^2) / (10xy - 4y^2), we first factor both the numerator and denominator.

Numerator: (25x^2 - 20xy + 4y^2)
= (5x)^2 - 2(5x)(2y) + (2y)^2
= (5x - 2y)^2

Denominator: (10xy - 4y^2)
= 2y(5x - 2y)

Now, we simplify the expression by dividing the factors:

= (5x - 2y)^2 / 2y(5x - 2y)
= (5x - 2y)(5x - 2y) / 2y(5x - 2y)
= (5x - 2y) / 2y

Therefore, the simplified form of the expression is (5x - 2y) / 2y.