She has already saved $85 which means the amount of money she needs left will be
=$415 - $85
= $330
Now n represents the number of lawns she needs to mow. She earns $15 per lawn, so she needs to multiply the cost of each lawn by the number of lawns (n) and equal it to the money she needs. This is how the equation will turn out.
15n = 330
To go further, divide both sides by 15 to get the number of lawns she has to mow.
n = 22
∴ She needs to mow 22 lawns to get the amount of money she needs.