ASM 12.14

[LoneGirl]

I am confused about how they got tp20 in this problem. I thought the definition of tpx was s(x+t)/s(x). So why wouldn't the numerator in this problem be (50/(70+x))^3 instead of (50/(50+x))^3?

[Abraham Weishaus]

You could use 70+t (where t is duration) or 50+x (where x is age): then you'd integrate t starting at 0 (instead of x starting at 20). It amounts to the same thing.