Answer: it will take them 8.89 hours
Explanation:
Amber can paint a room in 20 hours. This means that Amber's working rate per hour is 1/20
Christy can paint the same room in 16 hours. This means that Christy's working rate per hour is 1/16
If they work together, they would be working simultaneously and their unit rates is additive. This means that their combined rate would be
1/20 + 1/16
Assuming it takes t hours for both of them to paint the room working together. It means that their combined working rate per hour is 1/t. Therefore
1/20 + 1/16 = 1/t
(16 + 20/320 = 1/t
36/320 = 1/t
t = 320/36
t = 8.89 hours