PSLE
Natalie and Charlie bought jigsaw puzzles at the prices shown below.

3 Large jigsaw puzzles for $7
4 Small jigsaw puzzles for $5

1. Natalie bought an equal number of large and small jigsaw puzzles. She spent $39 more on the large ones. How many jigsaw puzzles did she buy altogether?
2. Charlie spent an equal amount of money on the large and small jigsaw puzzles. What fraction of the jigsaw puzzles he bought were large?

(a)
Number of large and small jigsaw puzzles that Natalie bought is the same. Make the number of large and small jigsaw puzzles the same. LCM of 3 and 4 is 12.

Cost of 3 large jigsaw puzzles = $7
Cost of 12 large jigsaw puzzles = 4 x 7 = $28

Cost of 4 small jigsaw puzzles = $5
Cost of 12 small jigsaw puzzles = 3 x 5 = $15

Difference in the cost of buying 12 small jigsaw puzzles and 12 large jigsaw puzzles
= 28 - 15
= $13

Number of sets of jigsaw puzzles
= 39 ÷ 13
= 3

Number of jigsaw puzzles that Natalie bought
= 3 x 2 x 12
= 72

(b)
Charlie spent an equal amount of money on the large and small jigsaw puzzles.  Make the amount of money spent on the large and small jigsaw puzzles the same.
LCM of 7 and 5 is 35.

Number of large jigsaw puzzles that Charlie bought
= 3 x (35 ÷ 7)
= 3 x 5
= 15

Number of the small jigsaw puzzles that Charlie bought
= 4 x (35 ÷ 5)
= 4 x 7
= 28

Fraction of the jigsaw puzzles that Charlie bought that were large
= ¹⁵_{15 + 28}
= ¹⁵₄₃

Answer(s): (a) 72; (b) ¹⁵₄₃