So basically 42=x times 64 or 42=64x where you want x to be represented as a fraction so just don't simplify as much
42=64x
divide both sdies by 64
42/64=64/64x
42/64=1x
x=42/64
simplify
42/64=21/32
the answer is 21/32 in simplest form
10, 20 , 30, 45 has 5 as their greatest common factor
