This is a combination problem. Order does not matter and the books are all different. You would write this as 21C5 or
. Combinations are not fractions.
The answer is 
which = 20349 different ways these books can be chosen.
The volume of B is twice that of A
if Vb=2hpir^2
and Va=hpir^2
compare
2hpir^2
hpir^2
twic as big
Volume of B is 2 times that of Volume of A
Line LC hope this helps out
The slope is rise/run
rise=up and down
run=left and right
the rise is 0
the run is inifnity
so 0/inifnity=0
the slope is 0