Roshel, William, Wyatt and Christopher share the same hobby of collecting coins. The average number of coins collected by the 4 friends is 21. William collected 7 more coins than Wyatt but 14 fewer coins than Roshel. Christopher collected 4 coins fewer than Wyatt. Find the average number of coins collected by William and Christopher. Round off your answer to the nearest whole number.
Number of coins Wyatt collected = 1 u
Number of coins William collected = 1 u + 7
Number of coins Roshel collected = 1 u + 7 + 14 = 1 u + 21
Number of coins Christopher collected = 1 u - 4

Total number of coins collected
= 4 x 21
= 84

Total number of coins collected in units
= 1 u + 1 u + 7 + 1 u + 21 + 1 u - 4
= 4 u + 24

4 u + 24 = 84
4 u = 84 - 24 = 60

1 u = 60 ÷ 4 = 15

Total number of coins collected by William and Christopher
= 1 u + 7 + 1 u - 4
= 2 u + 3
= 2 x 15 + 3
= 33

Average number of coins collected by William and Christopher
= 33 ÷ 2
≈ 17 (Round off to the nearest whole number.)

Answer(s): 17