Overall, 1, the first and second prizes account for 3/5, and the second and third prizes account for 7/ 10, so the second prize accounts for the total.
3/5+7/ 10- 1
= 13/ 10- 1
=3/ 10
Note "3/5+7/ 10" is the proportion of the second prize added to the overall first, second and third prizes, so the conclusion is drawn by subtracting the whole.