You walk in a straight line for 100m at 2 m/s. You walk straight back to where you started at 4 m/s. What is your average speed?

So, I know the answer is 2.67 m/s because I got the question wrong but I don't understand how I'm supposed to set this problem up.

I know average speed is distance/delta(t), and I think the distance is 200m, but I'm unsure how I can get the delta(t)...