PSLE
In a shop, candles are sold only in boxes. A box of 7 short candles costs $3.20 and a box of 5 long candles costs $5.90.

1. Tommy wants 28 short candles and 2 long candles for his lanterns. What is the least amount of money that Tommy will need to spend on the candles?
2. Erika bought 17 more long candles than short candles from the shop. The total number of candles she bought was fewer than 80. How much did Erika spend on the candles altogether?

(a)
Number of boxes of short candles that Tommy needs to buy
= 28 ÷ 7
= 4 

Amount that Tommy needs to spend on short candles
= 4 x 3.20
= $12.80

Least amount that Tommy will need to spend
= 12.80 + 5.90
= $18.70

(b)

Packets of long candles	Number of long candles  (x5)	Packets of short candles	Number of short candles (x7)	Total	Difference	Check
11	55	3	21	76	55 - 21 = 34	x
10	50	3	21	71	50 - 21 = 29	x
9	45	4	28	73	45 - 28 = 17	✓

To know the number of packets of long candles and short candles, we list the multiplies of 5 and 7.
We use systematic listing to find the difference between the number of long candles and short candles so that we can find the right combination that will result in having 17 more long candles.
We also check if the total number of candles must be less than 80.

Multiples of 5: 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80 
Multiples of 7: 7, 14, 21, 28, 35, 42, 49, 56, 63, 70, 77 

Number of packets of long candles = 9

Cost of long candles
= 9 x 5.90
= $53.10

Cost of short candles
= 4 x 3.20
= $12.80

Total amount that Erika spent on the candles
= 53.10 + 12.80
= $65.90

Answer(s): (a) $18.70; (b) $65.90