A 1050 page textbook needs to be typed in x hours. Lisa can type 15 pages in an hour and Whitney can type 12 pages in the same amount of time. Which of the following shows the expression that models this problem?
A: 1050/x = 12/x + 15/x
B: x/12 + x/15= x/1050
C: 15+12= x/1050
D: 27 = 1050/x