Shiyun was saving money to buy pens. She saved $3 every 6 days. After saving for 48 days, she went to buy pens that were on sale.

1. What was the most number of pens she could buy?
2. Shiyun wanted to buy 25 pens. How much more did she need to save?

(a)
Number of sets of 6 days
= 48 ÷ 6
= 8

Amount that Shiyun saved in 48 days
= 8 x 3
= $24

Maximum number of sets of 7 pens that Shiyun could buy 
= 24 ÷ 9
= 2 r $6

Remaining amount left after buying sets of 7 pens = $6

Maximum number of sets of 3 pens that Shiyun could buy with $6
= 6 ÷ 4
= 1 r $2 

Remaining amount left after buying sets of 3 pens = $2

Maximum number of pens that Shiyun could buy with $2 
= 2 ÷ 2
= 1

Most number of pens that Shiyun could buy
= 2 x 7 + 1 x 3 + 1
= 14 + 3 + 1
= 18

(b)
Number of sets of 7 pens that Shiyun could buy
= 25 ÷ 7
= 3 r 4

Number of sets of 3 pens that Shiyun could buy
= 4 ÷ 3
= 1 r 1

Number of individual pens that Shiyun could buy = 1

Amount that Shiyun needed to buy 25 pens
= 3 x 9 + 1 x 4 + 2
= 27 + 4 + 2
= $33

Amount that Shiyun needed to save more
= 33 - 24
= $9

Answer(s): (a) 18; (b) $9