You can use gauss's method
n(first number + last number)/2 where n is the number of integers in this case 21 ()
21(0.30 + 0.50)/2
21*0.8/2
21*0.4
8.4
The sandwich cost 2.25 meaning that that would be our slope since the price can go higher with every sandwich you buy.. and it will be negative because you will be taking away that sum to your five dollars.. so we have y=-2.25 +5 it has plus five because it is how much you had in the first place.. so lets test it, lets say you buy 2 sandwiches. you will put 2 in for x and solve it.. -2.25(2) +5 multiply -4.5+5 and you get .5 this lets you see first how much you will spend and what will be left out
Hope i helped
It’s 36x!! hope this helps
