Because you find the make the denominator to be the same, which is 4, then you keep 3/4 the same and change 1/2 to 2/4 because you times both the denominator and numerator. So it's 3/4 x 2/4 = 3/8.
4/7 x 21/40 = <u><em>3/10</em></u>
I already explained it and it's the same concept so yeah!
