crystal can wash 1/16 car per minute
sarawong can wash 1/15 car per minute
total they wash 1/16+1/15 or 15/(15*16)+16/(15*16)=(15+16)/(15*16)=31/240 car per minute
work times time=total 1 car
31/240 times time=1 car
times both sides by 240/31
time=240/31 minutes
time=7.7419354838709677419354838709677 min
about 7.74 minutes or about 7 and 3/4 minutes
