Allison can complete a sales route by herself in 6 hours. working with an associate, she completes the route in 4 hours. how long would it take her associate to complete the route by herself?

We know that
if Allison can complete a sales route by herself in 6 hours
then
100% of the sales route> 6 hours
x%> 1 hour
x=100/6> x=16.67%/hour

Allison working with an associate, she completes the route in 4 hours
then 
100%> 4 hours
x%> 1 hour
x=100/4> x=25%

An associate in 1 hour complete> 25%-16.67%> 8.33%
then
 an associate complete in 1 hour> 8.33%
 x hour> 100%
x=100/8.33> x=12 hours

the answer is
12 hours