A typist started typing the serial numbers of 600 students in a list, then how many times he has to press the numeric keys?180016891692600

The typist has to type the serial numbers of 600 students.

Let's break it down:

1. For students 1 to 9, he will press the numeric key once, so that's 9 keys.

2. For students 10 to 99, he will press the numeric key twice, so that's 90 students * 2 = 180 keys.

3. For students 100 to 599, he will press the numeric key three times, so that's 500 students * 3 = 1500 keys.

4. For student 600, he will press the numeric key three times.

So, in total, the typist will press the numeric keys 9 (for students 1-9) + 180 (for students 10-99) + 1500 (for students 100-599) + 3 (for student 600) = 1692 times.