Solving using 2 equations in two variable: Jason bought a total of 7 postcards for 1.80. If small cost 20 cents and the large ones cost 30 cents, how many postcards of each size did he buy?

Answer 1

S + L = 7

.2 S + .3 L = 1.8 ... S + 1.5 L = 9

Answer 2

The answer is 3 small and 4 large. I just do not know how to get there...L cannot equal 9 there are only 7 cards.

Thank you please help me know how to solve.

Answer 3

Eq1: S + L = 7.
Eq2: 0.2S + 0.3L = 1.8.
Multiply both sides of Eq1 by -0.2 and add the Eqs:
-0.2S - 0.2L = -1.4
0.2S + 0.3L = 1.8
Sum: 0.1L = 0.4
L = 4.

In Eq1, replace L with 4 and solve for S.