You want to change the fractions so they both have a common denominator. The lowest common multiple is 8. 1/8 is fine as it is, you want to change 3/4 so that it is over 8. You do this by multiplying 3 by 2 and 4 by 2. So you end up with 6/8. He reads 1/8 each night so it will take him 6 nights to read 6/8 of the book which is equal to 3/4 of the book.
