Hello!
The prime factors of these two numbres are :
\(25b^2 = 5^2\times b^2\\
20b = 5\times 2^2 \times b\)
The GCF can be calculated by multiplying common primes factors :
\(5\times b = 5b\)
So the GCF is 5b and the expression can be factored this way :
\(25b^2 - 20b = 5b\times 5b - 4\times 5b = 5b\left(5b-4\right)\)
