Liam, Risa, Raeann and Samantha share the same hobby of collecting marbles. The average number of marbles collected by the 4 friends is 20. Risa collected 8 more marbles than Raeann but 13 fewer marbles than Liam. Samantha collected 5 marbles fewer than Raeann. Find the average number of marbles collected by Risa and Samantha. Round off your answer to the nearest whole number.
Number of marbles Raeann collected = 1 u
Number of marbles Risa collected = 1 u + 8
Number of marbles Liam collected = 1 u + 8 + 13 = 1 u + 21
Number of marbles Samantha collected = 1 u - 5

Total number of marbles collected
= 4 x 20
= 80

Total number of marbles collected in units
= 1 u + 1 u + 8 + 1 u + 21 + 1 u - 5
= 4 u + 24

4 u + 24 = 80
4 u = 80 - 24 = 56

1 u = 56 ÷ 4 = 14

Total number of marbles collected by Risa and Samantha
= 1 u + 8 + 1 u - 5
= 2 u + 3
= 2 x 14 + 3
= 31

Average number of marbles collected by Risa and Samantha
= 31 ÷ 2
≈ 16 (Round off to the nearest whole number.)

Answer(s): 16