Li has to read four books from a list of 10 books. how many sets of four books can be made?

To answer this question, we would use the combination formula which is 
nCr, where n is the the number of objects and r is the sample.

So n would be 10, and r would be 4, since that is what we need to group.

10 choose 4 is written 10C4, or C(10,4), and equals 10!/(4!(10-4)!)=10!/(4!6!)=10*9*8*7/(1*2*3*4) = 210