200 +6m = 500-6m
add 6m to each side
200+12m = 500
subtract 200 from each side
12m = 300
divide both sides by 12
m = 300/12 = 25
25 minutes
check:
200+ 25*6 = 200 +150 = 350
500 - 25*6 = 500-150 = 350
