Hot air balloon 1 is 10 meters above the ground, rising 15 meters per minute, and hot air balloon 2 is 150 meters above the ground descending 20 meters per minute.
in how many minutes will the balloons be at the same height?
how high will the balloons be at that time?
(let x represent the time and y represent the height)

You have two equations
y = 10 + 15x   and    y = 150 - 20x
since  y = y   you can get
10 + 15x = 150 - 20x    now we solve for x
      +20x         +20x     add 20x to both sides
10 + 35x = 150    now subtract 10 from both sides
-10             -10
35x = 140    then divide both sides by 35
/35      /35
so  x = 4 minutes

to find what high they are at 4 minutes
plug 4 back into the equation for x
y = 10 + 15(4) = 10 + 60 = 70
so  70 meters from the ground