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11-29.

An infinite geometric series is of the form $a + \frac { 2 a } { 3 } + \frac { 4 a } { 9 } + \frac { 8 a } { 27 } + \ldots$ If the sum of the series is $90$, what is the value of $a$?

$\frac{\textit{a}}{1-\frac{2}{3}}=90$