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Answer:
19
Explanation:
it is 19 because, if you have 10 marbles, and then your friend has 9 and gives it to you, then you go from 10 then 11 then 12 then 13 then 14 then 15 then 16 then 17 then 18 then 19.
21
9 = 3 x 3
10 = 5 x 2
3 x 2 = 6
3 x 5 = 15
15 + 6 =21