A man is 45 years old today and his two sons are 15 and 9 years old. In how many years will the man's age equal the sum of the ages of his two sons?

After $t$ years the man is $45+t$ while the sons total $(15+t)+(9+t)=24+2t$. Setting $45+t=24+2t$ gives $t=21$. Check: in 21 years he is 66 and the sons are 36 and 30, which add to 66. 10.5 is the trap of holding the father's age still $(45=24+2t)$ - everybody ages together, and the sons' total climbs 2 years for every 1 of his. 24 is the sons' present total. 66 is the man's age at that time, not the wait.