Chapter 2 - Real Numbers - Chapters 1-2 Cumulative Review Problem Set - Page 91: 22

$2\times3\times13$

First, we divide 78 by the prime number 2. This results in the number 39. 39 is then divided by the next smallest prime number, 3, which results in 13. 13 is also a prime number. Therefore, as a product of prime factors, 78 can be written as: $78=2\times39=2\times3\times13$