國華在超級市場花了 111 元買橙、蘋果和芒果。橙每個 2 元，蘋果每個 3 元，芒果每個 5 元。如果他買的橙、蘋果和芒果的數目的和是 32 個，蘋果數量是橙數量的 2 倍，那麼他買的三種水果各多少個？

小學解法：
橙買一個就要買蘋果兩個，等於八元買三個，此稱一套。
假設買十套+芒果兩個=三十二個水果，需10x8+10=90
然後明顯可知如果少買一套，反而去買三個芒果這樣要多花15-8=7元
所以就(111-90)/7=3
少買三套就是7套，芒果加3x3=9個

ans.(7,14,11)

好啦我只是想推廣雞兔法解三元聯立而已

設橙買x個，蘋果y個，芒果z個
2x+3y+5z=111
x+y+z=32
y=2x

解得x=7，y=14，z=11
所以國華橙買了7個，蘋果買了14個，芒果買了11個

a=# of apples
m=# of mangoes
o=# of oranges
Given a=2o and a+m+o=32
So for every orange that was bought, two apples were bought.
Total cost of 1 orange and 2 apples (3 fruits per set)
=2+2(3)=8
Assume there are x sets of 1 orange and 2 apples, then

m+3x=32...............(1)
5m+8x=111............(2)
5m+15x=160..........5(1)
7x=49.................5(1)-(2)
x=7
Therefore
Kwok Wah bought 7 oranges, 14 apples, and 32-21=11 mangoes.
Check: 7(2)+14(3)+11(5)=14+42+55=111 .... good!