I need to find the number of aliens that have $3$ eyes, so I can multiply the fraction of them by the total number of aliens. I can use a calculator or mental math to do this. Multiplying $\frac{17}{40}$ by $160$, I get $\frac{17}{40} \times 160 = 17 \times 4 = 68$. Therefore, $68$ aliens had $3$ eyes. 
When a spaceship full of scientists landed on Planet Q, they found that $\frac{17}{40}$ of the $160$ aliens had $3$ eyes.  How many aliens had $3$ eyes?