Answer:
10
Step-by-step explanation:
Richard gets 15 sweets which is 3 parts of the ratio.
Dividing 15 by 3 gives the value of one part of the ratio
15 ÷ 3 = 5 sweets ← value of 1 part of the ratio, thus
3 × 5 = 15 ← number of sweets Henry gets
5 × 5 = 25 ← number of sweets Gavin gets
Thus
25 - 15 = 10 sweets which is how many Gavin gets over Henry