WAEC 2014 Mathematics · Question 39

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?

Options

Answer: D. 9

Explanation written with AI and checked against the answer key

Analyze the question: There are 13 notes in total, each either N50 or N20, and their total value is N530. We need the number of N50 notes. Concept: This is a simultaneous equations problem. Let one unknown be the number of N50 notes and the other the number of N20 notes. Solution: Let x = number of N50 notes and y = number of N20 notes. Number of notes: x + y = 13 ... (i) Total value: 50x + 20y = 530 ... (ii) Divide (ii) by 10: 5x + 2y = 53 ... (iii) From (i), y = 13 − x. Substitute into (iii): 5x + 2(13 − x) = 53 5x + 26 − 2x = 53 3x = 27 x = 9 So y = 13 − 9 = 4. Check: 9 × 50 = N450 and 4 × 20 = N80. Then 450 + 80 = N530 ✓, and 9 + 4 = 13 notes ✓. Answer: There are 9 N50 notes, which is option D. Exam tip: In "two types of items" problems, you can test the options. Plug in each option, find the remaining count (13 − x), and see which gives the total. Here, option A (4) is the number of N20 notes, so watch out for that trap and make sure you answer what the question asks.

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