Ratios, rates, proportional relationships, and units SAT question

Choice A is correct. It’s given that Tilly earns $p$ dollars for every $w$ hours of work. This can be represented by the proportion $\frac{p}{w}$. The amount of money, $x$, Tilly earns for $39w$ hours of work can be found by setting up the proportion $\frac{p}{w}=\frac{x}{39w}$. This can be rewritten as $39pw=xw$. Dividing both sides by $w$ results in $x=39p$. 

Choice B is incorrect. This is the amount of money Tilly earns in dollars per hour, not the amount of money Tilly earns for $39w$ hours of work.

Choice C is incorrect. This is the amount of money Tilly earns for $w$ hours of work plus $39$, not the amount of money Tilly earns for $39w$ hours of work.

Choice D is incorrect. This is the amount of money Tilly earns for $w$ hours of work minus $39$, not the amount of money Tilly earns for $39w$ hours of work.