The number of large boxes is 4 and the number of small boxes is 14.
<u><em>Explanation</em></u>
Suppose, the number of large boxes is and the number of small boxes is
As, the total number of boxes is 18, so the first equation is....
The large boxes hold 35 books, and the small ones hold 20 books and the total number of books is 420. So the second equation will be .....
Now solving equation (1) for itself , we will get....
We will plug this into equation (2). So.....
Now plugging this into the equation
.......
Thus, the number of large boxes is 4 and the number of small boxes is 14.