Question 1101866
<br>
The other tutor's answer is right.<br>
Here is my solution, by a slightly different method that looks easier than what they did.<br>
As the other tutor showed in her calculations, the man and the younger son can do 11/60 of the job in 1 hour.<br>
When the older son joins them, the job is finished in another 3 hours.<br>
That means the man and his younger son work for 4 hours, and the older son works for 3.<br>
Since the man and the younger son do 11/60 of the job in 1 hour, they do 44/60 of the job in 4 hours.<br>
That means the older son does 16/60 of the job in 3 hours, which means 16/180 of the job in 1 hour.<br>
So the number of hours the older son alone would need to do the job alone is 180/16 = 11.25 hours.