The vending machine has been used 30 times, so it has failed to drop a snack 30 / 6 = <<30/6=5>>5 times.
The vending machine has dropped 2 snacks 30 / 10 = <<30/10=3>>3 times.
It has operated normally and dropped 1 snack 30 - 5 - 3 = <<30-5-3=22>>22 times.
Thus, the vending machine has dropped 22 + 3 * 2 = 22 + 6 = <<22+3*2=28>>28 snacks in thirty uses.
#### 28