The answer is B. 3
It is the only value not attached to a variable, thus remaining constant.
She reads at 55 pages per hour for 4 hours so 55*4 = 220 pages so far. This means that she read 220 and had 330 left so 220 + 330 = 550 so the book is 550 pages. If she reads 55 pages per hour and there is a total of 550 pages, 550/55 = 10 hours to read the book.
All you have to do is divide the left side of the ratio by 5 and the right side of the ratio by 7. The one that comes out even on both sides is the correct one.
