The series: \begin{align}\sum\limits_{n=1}^{\in fty} \frac{n} {2^n}\end{align} ???

Can someone me out here number 4 plz

You're locked out of your house. the only open window is on the second floor, 25 feet above the ground. there are bushes along the edge of the house, so you will need to place the ladder 10 feet from the house. what length ladder do you need to reach the window?

Susie wants to take her friend out for lunch.she wants to spend $9.75 each person. how much will she spend in all.