Jill had some marbles in a bag. After Jay gave
her 7 more, she had 12 marbles in all How many
marbles were in the bag?
Jose wrote the equation M + 7 = 12.
He used a balance scale model to solve it.
Let M represent the marbles Jill originally had.
A
+
7
He took 7 marbles off each side.
Jill had 5 marbles in the bag.
Illustrate Jose's method in solving the following
problem: Bill had six books. Teruko gave him
some more, and then he had 10 books. How many
books did Teruko give Bill?

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