So first we will add together the fractions. 3/4 is equal to 6/8, so we will add 6/8 to 7/8 and get 13/8, or 1 5/8. Then we can add together the whole numbers, 19 and 18, to get 37. Then add 37 + 1 5/8 and get 38 5/8. Lastly, we subtract that amount from 50 to get a finale number of 11 3/8. The pet store donated 11 3/8 pounds of cat food! Hope this helps!