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Question:

The formula h -15 = 3.2t gives the height h in inches of a plant t weeks after planting. When was the height 31 inches?

A Sometime between 3 and 7 weeks
Explaination

h - 15 = 3.2t gives the height of a shrub in inches after t weeks. When the height is 31 inches 31 - 15 = 3.2t
3.2t = 16
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t = 5
The height is 31 inches after 5 weeks.