The average age of a group of $8$ people is $20\text{years}$. One person leaves the group and the average age becomes $18\text{years}$. Find the age of the person who left.

1. Total age of the initial group of $8$ people:
$${\text{Total Age}}_8=8\times 20=160\text{years}$$

2. When $1$ person leaves, $7$ people remain, and their new average age is $18\text{years}$. Total age of the remaining $7$ people:

$${\text{Total Age}}_7=7\times 18=126\text{years}$$

3. The age of the person who left is the difference between the two total sums:

$$\text{Age of the person who left}=160-126=34\text{years}$$