Question

how to solve this question?

Answer 1

let the number of 10rs. notes be x
and number of 20rs. notes be y

then

From given condition,

10x+20y=800...............,...................1

From second condition,

x+y=50..............................................2

Multiply equation 2 by 10 in order to make coefficient same of x in equation 1 and 2

10(x+y)=10(50)
10x+10y=500......................................3

Subtract equation 3 from 1

(10x+20y)-(10x+10y)=800-(500)

10x+20y-10x-10y=300
20y-10y=300
10y=300
$y = \frac{300}{10} \\ y = 30 \\ \\ substitute \: this \: value \: in \: any \: equation \: to \: find \: the \: value \: of \: x \: but \: always \: in \: the \: easier \: one \\ thus \: substitute \: y = 30 \: in \: equation \: 2 \\ x + y = 50 \\ x + 30 = 50 \\ x = 50 - 30 \\ x = 20$
thus the number of 10rs notes is 20 and of 20rs notes is 30