$C(t)= 30t+90$, $7$ months
Work Step by Step
Let $t$ = months of membeship. Then $C(t)= 30t+90$ and the t for which $C(t)=300$ is found from $300= 30t+90\qquad$ ... subtract 90 $ 210=30t\qquad$ ... divide with 30 $t=7$ months
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