Your grandmother gave you $10 with which to buy a present. this coverd 2/5 of the cost. how much did the present cost?

10 is the part but you are looking for the whole so set up a ratio

10/x=2/5

10 part x is whole

2 is part 5 is whole

now cross multiply the numerator 10 by the denominator 5 10x5=50 now divide 50/2 (the 50 you got by the numerator)

25 is the present's cost

The present costs $25.