$1+a+a^2+a^3+...+a^{x-1}+a^x=(1+a)(1+a^2)(1+a^4)(1+a^8)$ .

$\frac{x}{15}=?$

The answer is 1.

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multiply both the sides by ( 1- a) thus we get $1 -a^{x+1} = 1 - a^{16}$ thus x=15