Answer:
66 °F
Explanation:
At 4 P.M., the temperature had decreased by 3° from the temperature at 3 P.M., so was ...
4 P.M. temp = 78 °F -3° = 75 °F
This was repeated until 7 P.M.
5 P.M. temp = 75 °F -3° = 72 °F
6 P.M. temp = 72 °F -3° = 69 °F
7 P.M. temp = 69 °F -3° = 66 °F
The temperature at 7 P.M. was 66 °F.
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Of course the repeated subtraction can be simplified using multiplication. The subtraction of 3° four times is equivalent to the subtraction of . . .
(3°)(4) = 12°
That is, the temperature at 7 P.M. is ...
78 °F -3°×4 = 78 °F -12° = 66 °F