iti bought \( \frac{2}{5} \mathrm{~kg} \) of mushrooms. Bala bought \( \frac{1}{10} \mathrm{~kg} \) more mushroom than Siti. Vhat was the mass of mushrooms that they bought altogether?

Siti bought \( \frac{2}{5} \) kg of mushrooms. Since Bala bought \( \frac{1}{10} \) kg more than Siti, we first need to convert \( \frac{2}{5} \) into a fraction with a denominator of 10 to add the two amounts easily. \( \frac{2}{5} \) is equivalent to \( \frac{4}{10} \). Now, adding Bala's additional amount: \[ \text{Bala's mushrooms} = \frac{4}{10} + \frac{1}{10} = \frac{5}{10} = \frac{1}{2} \text{ kg}. \] Now, we add the amounts they both bought together: \[ \text{Total} = \frac{2}{5} + \frac{1}{2}. \] To add these, convert \( \frac{2}{5} \) into a fraction with a denominator of 10: \[ \frac{2}{5} = \frac{4}{10}, \quad \frac{1}{2} = \frac{5}{10}. \] So the total is: \[ \text{Total} = \frac{4}{10} + \frac{5}{10} = \frac{9}{10} \text{ kg}. \] Therefore, Siti and Bala bought \( \frac{9}{10} \) kg of mushrooms altogether.