Answer:
x < 15.6
Step-by-step explanation:
She spends on rent and electricity
P(x) = 8x + 15
She spends on rent
Q(x) = 3x - 7
She spends on electricity
R(x) = P(x) - Q(x) = (8x + 15) - (3x - 7) = 5x + 22
R(x) = 5x + 22
If R(x) < $ 100 we get
5x + 22 < 100
⇒ 5x < 78
⇒ x < 15.6