I don't know your options, but here are some facts:
-all their options are sandwiches, and they all cost 5 dollars.
-This means that they can buy 150/5=30 sandwiches. This will make them use the entire budget!
-I would suggest cutting the sandwiches in half, then everyone gets three halves!
and if you say r for roast beef and t for tuna, you can write:
r+t=30.
and you can distribute is freely as long as both are 0 or bigger and their sum is 30!