Al’s job rate is 1 job in 25 minutes or 1/25.
Joe’s job rate is 1 job in J minutes or 1/J.
Together their rate is 1/25+1/J=one job in 15 minutes=1/15.
So the equation is 1/25+1/J=1/15.
Multiply through by 75J:
3J+75=5J so 2J=75 and J=37.5 minutes, if Joe did the job on his own.