When 2 people work the formula is:
Person1 * Person2 / (Person1 + Person2)
rob becomes Person1 and Kayla becomes Person 2
9 * K / (9 + K) = 4.74 Hours
9*K = 4.74*9 + K*4.74
9*K = 42.66 + K*4.74
4.26 *K = 42.66
K = 10.014084507 hours (Kayla)
TESTING the answer
(9 * 10.014084507) / (9 + 10.014084507) = 4.74
90.126760563 / (19.014084507) = 4.74
4.74 = 4.74
