Step-by-step explanation:
to divide by a fraction is the same as to multiply by the upside-down fraction.
in your example
4 / 2/5 = 4 × 5/2 = 20/2 = 10
by the way, also a whole number is a fraction.
4 = 4/1
so, fully formal, the whole operation above looks like
(4/1) / (2/5) = (4/1) × (5/2) = (4×5) / (1×2) = 20/2 = 10/1 = 10
