I am having difficulty with the following problem:

Note : the ans is 13.33Mr.Boone can do a job in 10 days . A helper joins him after 3 days and together they work for 4 days to complete the task.How many days would it take the helper to do the task alone ?

I know the formula for time taken for two people when working together is

T = 1/ ( (1/x) + (1/y))

But i cant manage to compensate mr boon for the 3 days he worked on his own. How would i do that ??