Answer:
$1.42
Explanation:
let each postcard cost $P and each envelope cost $E
given that the first customer paid $12 for 14 postcards and 5 envelopes.
Mathematically....
14P + 5E = 12 ---------(eq1)
similarly, the second customer paid $24.80 for 10 postcards and 15 envelopes.
10P + 15E = 24.80 ------- (eq 2)
Solving the system of equations by elimination:
(eq 1) x 3:
3(14P + 5E) = 3(12)
42P + 15E = 36 ------(eq 3)
eq3 - eq2
(42P + 15E) - (10P + 15E) = 36 - 24.80
42P + 15E - 10P - 15E = 11.20
42P - 10P = 11.20
32P = 11.20
P = 11.20 / 32 = $0.35
substituting this back into eq 1
14P + 5E = 12
14(0.35) + 5E = 12
4.9+ 5E = 12
5E = 12 - 4.9
5E = 7.1
E = 7.1/5
E = $1.42 (answer)