Add the two amounts by converting to improper fractions with a common denominator.<br><br>
&bull; \(3\frac{1}{3} = \frac{10}{3}\) and \(3\frac{7}{9} = \frac{34}{9}\)<br>
&bull; LCM of 3 and 9 is 9.<br>
&bull; \(\frac{10}{3} = \frac{30}{9}\)<br>
&bull; \(\frac{30}{9} + \frac{34}{9} = \frac{64}{9}\)<br>
&bull; Convert to mixed number: \(\frac{64}{9} = 7\frac{1}{9}\)<br><br>
They bought \(7\frac{1}{9}\) kg of rambutans altogether.<br>
Check: \(7\frac{1}{9} - 3\frac{7}{9} = 6\frac{10}{9} - 3\frac{7}{9} = 3\frac{3}{9} = 3\frac{1}{3}\). ✓