To go to sleep, Oliver counted by 6s starting from 15 . Thus the first few numbers he named were 15,21,27,33....
The last number he named before he fell asleep was 363. How many numbers did Oliver name?
/* Number of terms of arithmetic series */
(L - F) / D + 1, where L= Last term, F = first term, D = Common difference
(363 - 15) / 6 + 1 = 59 terms.
Another solution is presented here. https://softmath.com/algebra-word-problems/to-go-to-sleep-oliver-counted-by-6s-starting-from-15-thus