The probability that a student lives within 3 miles of the school is 75%, and the probability that a student who lives within 3 miles walks to school is 20%. We can find the probability that a student both lives within 3 miles and walks to school by multiplying these probabilities:
0.75 x 0.20 = 0.15
Therefore, the probability that a student lives within 3 miles and walks to school is 0.15 or 15%.