try to look for question data first

no constraints on integers - they can be repeated (not unique), just note this is useful for many questions involving numbers, integers their combination and arrangement for the future, as you request approach anyway

then rewrite in algebraic means the question's data, which is easily conveyed if we consider the following

n(n+1) and 15<n(n+1)<200

Again since you are looking for integers (not any value) only the whole numbers would be counted as your correct choices. Then solve inequalities for both sides

n(n+1)>15 and n(n+1)<200

n cold be 4 and 13

test min): 4(4+1)>15 OK
test max): 13(13+1)<200 OK

c

Thanks. That helped.