Each deli sandwich made uses 1/4 pound of roast beef. marisa started with 5 pounds of roast beef and now has 3/4 pounds left.

how many deli sandwiches did she make?

Let us assume the number of sandwiches made by Deli = x
Amount of roast beef required for making 1 sandwich = (1/4) pound
Total amount of beef that Marisa started with = 5 pounds
Amount of beef that was left = (3/4) pounds
Then
[(1/4) * x] + (3/4) = 5
(x/4) + (3/4) = 5
(x + 3)/4 = 5
x + 3 = 5 * 4
x + 3 = 20
x = 20 - 3
   = 17
So we can see from the above deduction that 17 Deli sandwiches was made by Marisa. I hope the procedure is clear enough for you to understand.