Example There are 3 bags of marbles in the left-side pan of a
balance. Each bag has the same number of marbles.
After you put 9 marbles in the right-side pan, the pans
hang evenly. How many marbles are in each bag?
Draw a picture to represent the problem.
Write an expression for the number of marbles in
each pan.
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number of marbles in left-side pan: 3x
number of marbles in right-side pan: 9
Write an equation to show that the expressions
are equivalent: 3x = 9.