You can use a system of equations to solve this problem.
Let s be small drinks, and l be large drinks.
3s+5l=3125
s+l=735
You can use substitution, elimination, or graphing to solve.
Elimination:
3s+5l=3125
To make the l coefficients the same, multiply the bottom equation by 5:
3s+5l=3125
5s+5l=3675
Subtract: -2s=-550.
275 small drinks were sold.
Substitution:
3s+5l=3125
s+l=735.
Isolate l on one of the equations: l= 735-s.
Substitute: 3s+(3675-5s.)=3125.
-2s+3675=3125.
-2s=-550
s=275.
