Apet shop has rabbits and gerbils for sale. they have 20 animals altogether. they sell two rabbits. then one of the gerbils has twins. the shop now has three times as many gerbils as rabbits. how many of each did they start with? (how do you get the answer? )

20 animals - 2 rabbits = 18 animals
18 animals + 2 gerbils = 20 animals.
20 animals / 4 = 5 animals
There are 5 rabbits and 15 gerbils now.
At the start 2 rabbits were sold so add that on.
5 + 2 = 7
There were 7 rabbits.
There weren't any twin gerbils then.
15 - 2 = 13.
There were 13 gerbils and 7 rabbits to start with.

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21/2 or 5/2