Below is an instruction that describes a task. Write a response that appropriately completes the request in English. Please answer in English.

### Instruction:
4 students participated in a 200m race. If the average completion time of the last three students was 35 seconds, and the average completion time of all four runners was 30 seconds, how long (in seconds) did it take the student who came first to finish the race?

### Response:
The last three students had an average completion time of 35 seconds so the sum of their completion times was 3*35 = <<3*35=105>>105 seconds
All four students had an average completion time of 30 seconds, which is the same as the first student's completion time, so the sum of their completion times is 4*30 = <<4*30=120>>120 seconds
The first student's completion time is 120-105 = <<120-105=15>>15 seconds
#### 15