Answer:
The distributive property states that
a*(b + c) = a*b + a*c
You can use the distributive property in reverse to factor out a common factor from the sum of 2 numbers. If you realize that 12 = 4*3 and 20 = 4*5 then you can write
12 + 20 = 4*3 + 4*5 = 4*(3 + 5)
