For clearly look we can seperate the number parts and the letter parts because they are all “*” and “/“ caculation
Initial fraction: (20 * a^2 * b) / ( 5 * a * b^2) = (20 / 5) * (a^2 / a) * (b / b^2)
Result: (4*a) / (b) = (4*a) / (1*b) = (4/1) * (a/b)
As you can see the result is a fully simplifying form so just simplify any thing you can from initial fraction
20/5 = 4/1 (both nume and deno are divided by 5)
a^2 / a = a/1 (divided by a)
b / b^2 = 1/b (divided by b)
Multiply all above we get the result 4a/b (just for checking whether we did right or not)
Next, multiply all that we have divided in previous step: 5*a*b = 5ab
=> problem solved