Answers:
Q: What is the most they could weigh together?
A: 0.74 kg
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Q: What is the least they could weigh together?
A: 0.62 kg
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Work Shown:
x = weight of first ball
y = weight of second ball
each ball has a weight range of 0.31 kg to 0.37 kg, so,
add straight down to get
which simplifies to
the two soccer balls have a weight range of 0.62 to 0.74, inclusive of both endpoints.
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Without using algebra, you basically just add the smallest the two weights could be (0.31) to itself to get 0.31+0.31 = 0.62 which represents the smallest the two weights combined can be. The same happens with the largest weight of 0.37 to get 0.37+0.37 = 0.74 as the max weight of both objects together.