10: 45pm-12: 15am = 6 3/4 pages
how many pages per hour?

Let's break apart the problem and solve it in bits.

10:45pm - 12:15am = 90 mins (1hr30)

So in a span of an 1hr and 30 minutes, she read 6.75 pages. To solve how many pages read per hour, set up a proportion and cross multiply:

90/6.75 = 60/x  multiply 90 and "x" to get 90x, and 6.75 and 60 to get 405. We need to isolate the variable, and in order to do that, you must use the opposite operation of what the "x" is involved in. Here we have 90x ("x" is multiplied to 90) so we must divide. Divide 90 from 90x to cancel out the 90 and leave us with just "x".

What you do on one side, you must do to the other, so divide 405 by 90 to get 4.5.

x=4.5, or there were 4.5 pages read in an hour. 

Aand b are correctly balanced reactions. they have the same number of atoms in both sides of the reaction