Kevin begins the day with 15 phones and repairs 3 by the afternoon, so Kevin still has to repair 15 - 3 = <<15-3=12>>12 phones.
A client drops off 6 additional phones that require fixing, increasing the total to 12 + 6 = 18 phones.
If Kevin's coworker helps Kevin fix half of the phones, then each person needs to fix 18 / 2 = <<18/2=9>>9 phones.
#### 9