How To Figure Out The Problems In Poptropica Game How Do I Figure These 7th Grade Problems Out?

How do i figure these 7th grade problems out? - how to figure out the problems in poptropica game

I do not understand!
The probability and combinations, is an example.
Suppose that 100 people a shovel buyback in 2007, how many people buy in a can wait in December?

I'm confused, can someone help me how to do it?