Hi, I'm happy to help!
First off, we need to arrange an equation for what is being asked. 15 is 20% of her earnings and we don't know her total earnings.
15=20%×__
We can write this percentage as
15=0.2×__
We then change the equation to do the inverse operation, which is division, this makes it easier to solve.
15÷0.2=__
Now, we solve
15÷0.2=75
She earned $75
I hope this was helpful, keep learning! :D