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Which is an equation for the nth terms of the sequence 12,15,18,21

Acyclist bike x distance at 10 miles per hour .and returns over the same path at 8 miles per hour. what is the cyclist average rate for the round trip in miles per hour ?

The scatter plot shows the number of animal cells clara examined in a laboratory in different months: plot ordered pairs 1, 20 and 2, 60 and 3,100 and 4, 140 and 5, 180 what is the approximate predicted number of animal cells clara examined in the 9th month?