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Jackie's Column Method for Division
I can solve the problem by drawing one column for each cup and “filling” them up with numbers. I'll start by putting 10 rocks into each cup. That uses up 50 rocks. Now I have 37 rocks left in the bag, since 87 – 50 = 37.
I'll put 5 more rocks in each cup to use 25 more rocks. That leaves 12 rocks still to divide, since 37 – 25 = 12.
Now I put 2 more rocks in each cup. That adds up to 10. 12 – 10 = 2, so there are 2 rocks still in the bag.
Each person gets 10 + 5 + 2 = 17 rocks, with 2 left over.