We need to use the following formula of distance .
Distance= speed * time .
For the first time
distance = 6r
And let the average rate for the second time be s
So distance = 5s
Since distance remains same, so we get
6r=5s
Now we have to solve for s. And for that we need to get rid of 5, by dividing both sides by 5, that is

And that's the required average rate for the second trip .