Just work the problem in reverse.
1:14pm - 24 minutes would be 12:50pm.
12:50pm - 15 minutes would be 12:35pm.
Jennifer left her house at 12:35pm.
if PLAN A is cheaper than PLAN B then,
P (A) < P (B)
30+0.01t < 20+0.05t (subtract 0.05t from both sides)
29.95-0.04t < 19.95 (subtract 29.95 from both sides)
-0.04t < -10 (divide both sides by -0.04)
t > 250
So plan A is cheaper than plan B when you send more than 250 texts.
Hope it helps.
They are reflected across the y-axis.
