You can put this solution on YOUR website!
Let x=amount of time it takes Charles alone to string the same line
Then Charles works at the rate of 1/x of the line per day
John can string 1/3 of the line per day
Together John and Charles can string 1/x + 1/3 of the line per day
But we are told that together they can do the job in 1 day, so:
1/x + 1/3 =1 multiply each term by 3x
Hope this helps---ptaylor