【Question 6】
There are two sets of numbers, the sum of the first set of 7 numbers is 84, the average of the second set is 21, and the average of all numbers in both sets is 18, then the second set has
$\qquad$ numbers.
【Analysis and Solution】
(Method One)
Let the second group have $x$ numbers;
$$
84+21 x=18(7+x) ;
$$

Solving for $x$ gives $x=14$;
The second group has 14 numbers.
(Method Two)
The average of the first group is $84 \div 7=12$; Cross method:

First Group
Second Group
12
21
18
;
$$
\begin{array}{ll} 
& / \text { } \\
3 & \\
1 & \\
6
\end{array}
$$

The ratio of the number of elements in the first group to the second group is $1: 2$;
The second group has $7 \div 1 \times 2=14$ numbers.