Question 869447
Crew 1 takes 4 hours so their rate is (1/4)auditorium per hour
Let x be the rate in auditoriums per hour of Crew 2
( 1/4 + x )( 2 1/2 hours ) = one clean auditorium
( 1/4 + x )( 5/2 ) = 1
multiply each side by 2/5
( 1/4 + x ) = 2/5
add -1/4 to each side
 x = 2/5 -1/4
 x = 8/20 -5/20
 x = 3/20
3/20 represents 3 auditoriums per 20 hours
or 1 auditorium every 6 2/3 hours ( divide 20 by 3 to get this )