PSLE
Sarah baked some cookies. She gave ¹₅ of the cookies to her friends and 72 to her students. She was left with ¹₂ of the cookies. She packed these into 16 boxes. Some boxes contained 5 cookies while the rest contained 9 cookies.

1. How many cookies were packed into the 16 boxes?
2. How many boxes contained 9 cookies?

(a)
Fraction of the cookies given to her students
= 1 - ¹₅ - ¹₂

= 1 - ²₁₀ - ⁵₁₀
= ³₁₀

Number of cookies that Sarah gave to her students = 3 u

3 u = 72

1 u = 72 ÷ 3 = 24

Number of cookies that were packed in 16 boxes
= 5 u
= 5 x 24
= 120

Number of cookies that were packed in 16 boxes = 120

Boxes of 9	Cookies	Boxes of 5	Cookies	Total cookies
16	144	0	0	144
15	135	1	5	140

If all the cookies are packed in boxes of 9, the number of cookies needed
= 16 x 9
= 144

Small difference
= 144 - 140
= 4

Big difference
= 144 - 120
= 24

Number of boxes of 5 cookies 
= 24 ÷ 4
= 6

Number of boxes of 9 cookies
= 16 - 6
= 10

Answer(s): (a) 120; (b) 10