let number of small sodas sold be x
# of large sodas sold be y
so
x + y =35
x=35 - y {equation1}
now next equation
0.65x + 0.45y = 18.55 { divide all by 0.50}
1.3x + 0.9y = 37.1 { put x = 35 - y (from eq 1)}
1.3 (35 - y) +0.9y = 37.1
45.5 - 1.3y + 0.9y = 37.1 {subtract 45.5 from both sides}
-0.4y = -8.4 { divide both by -0.4}
y = 21
so #of large sodas sold is 21
# of small sodas sold is 35 -21 = 14
