The number we are looking for is $800$.
Work Step by Step
We first translate this statement into an algebraic equation: "$120$ is" means "$120 = $." "15% " can be translated into a decimal, which would be $0.15$. "Of what number" means multiplying by $x$. We now put the whole statement together into the following equation: $$120 = 0.15x$$ We solve for $x$ by dividing each side by $0.15$: $$x = 800$$ The number we are looking for is $800$.