A sum of Rs 800 is in the form of denominations of Rs 10 and Rs 20.

Let the number of Rs. 10 notes be $\mathrm{x}$.

Therefore, the number of Rs. 20 notes will be $(50-\mathrm{x})$.

Value of Rs. 10 notes $=10 \mathrm{x}$

Value of Rs. 20 notes $=20(50-\mathrm{x})$

According to the question,

$10 \mathrm{x}+20(50-\mathrm{x})=800$

or $10 \mathrm{x}+1000-20 \mathrm{x}=800$

or $10 \mathrm{x}=1000-800$

or $\mathrm{x}=\frac{200}{10}=20$

$\therefore$ Number of Rs. 10 notes $=20$

Number of Rs. 20 notes $=(50-20)=30$.