The experimental probability of an event is calculated by dividing the number of successful outcomes by the total number of trials. In this case, the student flipped the coin 100 times and got heads 42 times.
So, the experimental probability \( P \) of flipping heads is:
\[ P(\text{Heads}) = \frac{\text{Number of heads}}{\text{Total flips}} = \frac{42}{100} = 0.42 \]
Therefore, the experimental probability of flipping heads is 0.42.