Wilma, a wicked witch, decided to steal 300 ladybugs for her garden. On the first day she took 20 ladybugs. On the second day she took 15 more ladybugs than on the first day. Each day she took 15 more ladybugs than she had taken the day before. How many days did it take Wilma the wicked witch to steal more than 300 ladybugs?
Recall the sum formula for APs (nothing to do with LCM, by the way)
You have an AP with
a = 20
d = 15
so you want n such that
n/2 (2*20+(n-1)*15) > 300
n > 5.54