Let C, M and S be the quantity of cups of coffee, muffins and sandwiches respectively.
C+M+S=55 (total items), C+2M+3S=96 (sales), M=C+6.
Therefore C+C+6+S=55, 2C+S=49; C+2(C+6)+3S=96, 3C+12+3S=96, 3C+3S=84, C+S=28, C=28-S.
Substitute for C: 2(28-S)+S=49, 56-S=49, S=7 sandwiches (C=21, M=27).