litterally you are lazy, search it up, https://math.stackexchange.com/questions/2104450/120-60-80-90
Sorry, what's the answer...?It's a terrible question unless you've missed out a key part of the question. There are multiple perfectly valid answers, mostly listed on the stack exchange post @Creepis linked.
Most obvious option is that fraction thing though: a_0=120, a_n = a_0(n/(n+1)) where n>1.
With some rather simple math, i get this:
∑n=1∞n3(132+103+32√+64+2x52+2x132103+32√√2242)n=42∑n=1∞n3(132+103+32+64+2x52+2x132103+322242)n=42