Will you help me with this (easy) maths question?

I am tutoring my sister. Both of us understand the question, we’ve solved it, and now we’re stuck.

Here it is:
Lanor and Matt are having a race on a 400m track. Lanor runs 220m/minute, Matt 180m. How long until Lanor laps Matt?

Now, obviously, the answer is 10 minutes. Lanor needs to run 400m more than Matt to lap him, and she covers forty extra metres per minute. So, 400/40, which is ten. What I can’t do is express this in an equation, explaining how I got there. Can anyone help?