Krystal washes the car in 16 minutes, so she can wash 1/16 of the car per minute.
Sara washes the car in 15 minutes, so she washes 1/15 of the car per minute.
Together they would wash: 1/16 + 1/15 = 31/240 of the car per minute.
To wash the entire car would be: 240/31 = 7.74 minutes ( 7 minutes 44.5 seconds)
