WAEC - Mathematics (2014 - No. 20)
A box contains 13 currency notes, all of which are either N50 or N20 notes. The total value of the currency notes is N530. How many N50 notes are in the box?
4
6
8
9
Spiegazione
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
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