Answer: at 15 months both Sue's card and Peggy's card have the same value
Explanation:
set up equations
Sue has the function S(x)
Peggy has function P(x)
S(x) = 75 - 2.5 x For x is in months
P(x) = 60 - 1.5 x
S(x) and P(x) represents value of Sue and Peggy's card after x months
S(x) = P(x)
75 -2.5x = 60 - 1.5x
15 = x
At 15 months, both Sue and Peggy have the same value on their gift cards
from MysticAlanCheng