To find out how much potting soil Henry uses, we need to calculate \( \frac{2}{5} \) of \( \frac{9}{10} \) kilogram of potting soil.
We can do this by multiplying the two fractions:
\[ \frac{2}{5} \times \frac{9}{10} = \frac{2 \times 9}{5 \times 10} = \frac{18}{50} \]
Now, we can simplify \( \frac{18}{50} \):
Both the numerator and the denominator can be divided by 2:
\[ \frac{18 \div 2}{50 \div 2} = \frac{9}{25} \]
So, Henry uses \( \frac{9}{25} \) kilograms of potting soil to plant the roses.