79. Let $x_{1}, x_{2}, \cdots, x_{7}$ be natural numbers, and $x_{1}<x_{2}<\cdots<x_{6}<x_{7}$, also $x_{1}+x_{2}+\cdots+x_{7}=159$, then the maximum value of $x_{1}+x_{2}+x_{3}$ is $\qquad$ .
Answer: 61