A box contains 13 currency notes, all of which are either N50 or N20 notes....

WAEC 2014

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?

A. 4
B. 6
C. 8
D. 9

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Correct Answer: Option D

Explanation

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

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