42 is 70% of what number?

Since "percent" or "%" refers to "out of 100" or "per 100," 70% can be expressed as ${\displaystyle \frac{70}{100}}$.
The word "of" means "times" or "to multiply" when discussing percents.
Finally, lets call the number we are looking for "n".
Putting this altogether we can write this equation and solve for n while keeping the equation balanced:
${\displaystyle 42=\frac{70}{100}\times n}$
${\displaystyle \frac{100}{70}\times 42=\frac{100}{70}\times \frac{70}{100}\times n}$
${\displaystyle \frac{4200}{70}=\frac{\cancel{100}}{\cancel{70}}\times \frac{\cancel{70}}{\cancel{100}}\times n}$
${\displaystyle 60=n}$
${\displaystyle n=60}$
42 is 70% of 60