Answer:
12.8 km + r = 31.4 km
Explanation:
First, Alex ran 12.8 km, then he ran an r amount of km. Added together they equal 31.4 km.
If you need to solve the equation you subtract 12.8 from 31.4 and will give you the amount of miles he ran after lunch.
Ex.: r = 31.4 km - 12.8 km
r = 18.6 km
Alex ran 18.6 km after lunch