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.