Simplify by combining like terms.
\[
\begin{array}{c}
4 \frac{2}{3} u+9 \frac{2}{9} u+8 u \\
{[?] \frac{[]}{[]} u}
\end{array}
\]
Remember to simplify the mixed number! Enter the number that goes in the green box.
Enter

Step 1 ：Convert the mixed numbers to improper fractions. The first term, 4 2/3, is equivalent to 14/3. The second term, 9 2/9, is equivalent to 83/9. The third term, 8, is equivalent to 8/1.

Step 2 ：Add these three fractions together. To do this, we need to find a common denominator, which is the least common multiple of the denominators 3, 9, and 1.

Step 3 ：The total of the three terms is 197/9.

Step 4 ：Convert the improper fraction back to a mixed number. The whole number part of the mixed number is the quotient of the division of the numerator by the denominator, and the fractional part is the remainder of the division.

Step 5 ：The simplified expression is \(21 \frac{8}{9} u\). The number that goes in the green box is \(\boxed{21}\).