Question

Jim paddles from one shore of a lake three miles wide at 4 mph, and John paddles from the opposite
shore at 5 mph. How long will they travel before they meet?
please explain to me

Answer 1

let Jim's distance be x miles
then John's distance is 3-x miles

Jim's time = x/4
John's time = (3-x)/5
when they meet, they will both have paddled the same time

so ...
x/4 = (3-x)/5
5x = 12 - 4x
9x = 12
x = 12/9 miles

so Jim's time = (12/9) /4 = 1/3 hour or 20 minutes

Answer 2

or, you can consider it like this. The boats are approaching each other at 9 mph. The lake is 3 miles wide, so it takes them 1/3 hour to cover all the distance.