40! / 30! = 40*39*38*37*36*35*34*33*32*31
dividing this by 10! 10*9*8*7*6*5*4*3*2*1
= 4*13*19*37*17*11*4*31
= 847,660,528
I think it’s 24 I could be wrong but I’m pretty sure
Answer:
What is countercurrent multiplication? ... Countercurrent multiplication in the kidneys is the process of using energy to generate an osmotic gradient that enables you to reabsorb water from the tubular fluid and produce concentrated urine.
We can compare by converting Johnny's reading time into the same unit of Jacky's, hours.
There are 60 minutes in an hour, so use this:

15 < 20, so Johnny read faster. This is 5 pages faster per hour than Jacky.