let the number of N50 notes = x
let the number of N20 notes = y
from the first statement
x + y = 13.....(1)
from the second statement
50x + 20y = 530 ......(2)
dividing through by 10, we have
5x + 2y = 53 ....(20
solving simultaneously by eliminating
x + y = 12....(1)
5x + 2y = 53.....(2)
multiply eqn (1) by 2
multiply eqn (2) by 1
2x + 2y = 26
-5x + 2y = 53
-3x = -27
x = 9
substitute x = 9 into equation (1)
x + y = 13
y = 13 - x
= 13 - 9 = 4
y = 4
the number of N50 notes = 9