Answer:
111 pounds
Step-by-step explanation:
A boy holding a stack of 9 identical books steps on a scale and weighs 138 pounds.
If the boy's weight is represented as B and the books are represented as b, this implies that:
B + 9b = 138
The boy then holding a stack of 3 of the same books again steps on the scale and weighs 120 pounds. This implies that:
B + 3b = 120
This has yielded two simultaneous equations:
B + 9b = 138
B + 3b = 120
We can solve by subtracting the second from the first.
B + 9b = 138
- (B + 3b = 120)
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=> 6b = 18
b = 18 / 6 = 3 pounds
This means that each book weighs 3 pounds.
Putting b back in the second equation:
B + (3 * 3) = 120
B + 9 = 120
B = 120 - 9
B = 111 pounds
Therefore, the boy weighs 111 pounds.