Question

If 4/5 x blank=2/5 what is blank

Answer 1

4/5 x 1/2 = 2/5

blank = 1/2

Answer 2

4/5x = 2/5
(5/4) 4/5 x = (5/4) 2/5 (multiplying both sides by 5/4 to isolate x)
x = 2/4 (5 in the numerator and 5 in the denominator cancels out)
x = 1/2 <--------- This is the answer.
Hope this helps.