Question 107174
Distance=rate*time or time=distance/rate & seeing as the rates are accumulative, we have: 
15/(2.5+3.5)=x now cross multiply.
6x=15
x=2.5 hours for them to meet.
Proof
2.5*2.5+3.5*2.5=15
6.25+8.75=15
15=15