lets assume that x is the number that we are multiplying by 4/5 and then divide by 2/5:

x*(4/5):(2/5) = x*(4/5)*(5/2) = x*((4*5)/(5*2)) = x*(20/10) = x*2

multiplying a number by 4/5 then divide by 2/5 is the same as multiplying by 2.