I do not understand why you or your teacher have written down a certain number of minutes by each equation, but I will do my best to answer your question. I hope that I’m able to help.
1,2) 20 + 3.5x = 30 + 2.75x
-20 -20
3.5x = 30 + 2.75x - 20
-2.75x -2.75x
0.75x = 10
/0.75 /0.75
x = 13.333...
3,4) 3.5x + 2.75x = 20 + 30
6.25x = 50
/6.25. /6.25
x = 8