Harper, Joshua, Luke and Christopher share the same hobby of collecting cards. The average number of cards collected by the 4 friends is 30. Joshua collected 9 more cards than Luke but 12 fewer cards than Harper. Christopher collected 2 cards fewer than Luke. Find the average number of cards collected by Joshua and Christopher. Round off your answer to the nearest whole number.
Number of cards Luke collected = 1 u
Number of cards Joshua collected = 1 u + 9
Number of cards Harper collected = 1 u + 9 + 12 = 1 u + 21
Number of cards Christopher collected = 1 u - 2

Total number of cards collected
= 4 x 30
= 120

Total number of cards collected in units
= 1 u + 1 u + 9 + 1 u + 21 + 1 u - 2
= 4 u + 28

4 u + 28 = 120
4 u = 120 - 28 = 92

1 u = 92 ÷ 4 = 23

Total number of cards collected by Joshua and Christopher
= 1 u + 9 + 1 u - 2
= 2 u + 7
= 2 x 23 + 7
= 53

Average number of cards collected by Joshua and Christopher
= 53 ÷ 2
≈ 27 (Round off to the nearest whole number.)

Answer(s): 27