The answer is 70 miles.
If we express distances as following:
T - total distance
a - distance from San Antonio to Austin
b - distance from Austin to Waco
c - distance from Waco to Dallas
Then:
T = 280 mi
b = a - 30 mi ⇒ a = b + 30
b = c + 20 mi ⇒ c = b - 20
T = a + b + c
⇒ a + b + c = 280
⇒ b + 30 + b + b - 20 = 280
⇒ 3b +10 = 280
⇒ 3b = 280 - 10
⇒ 3b = 270
⇒ b = 90 mi.
Distance from Waco to Dallas is c.
c = b - 20
⇒ c = 90 - 20
⇒ c = 70 mi
Therefore, distance from Waco to Dallas is 70 miles.
