Risa was saving money to buy pens. She saved $9 every 7 days. After saving for 21 days, she went to buy pens that were on sale.
- What was the most number of pens she could buy?
- Risa wanted to buy 42 pens. How much more did she need to save?
(a)
Number of sets of 7 days
= 21 ÷ 7
= 3
Amount that Risa saved in 21 days
= 3 x 9
= $27
Maximum number of sets of 9 pens that Risa could buy
= 27 ÷ 7
= 3 r $6
Remaining amount left after buying sets of 9 pens = $6
Maximum number of sets of 5 pens that Risa could buy with $6
= 6 ÷ 4
= 1 r $2
Remaining amount left after buying sets of 5 pens = $2
Maximum number of pens that Risa could buy with $2
= 2 ÷ 2
= 1
Most number of pens that Risa could buy
= 3 x 9 + 1 x 5 + 1
= 27 + 5 + 1
= 33
(b)
Number of sets of 9 pens that Risa could buy
= 42 ÷ 9
= 4 r 6
Number of sets of 5 pens that Risa could buy
= 6 ÷ 5
= 1 r 1
Number of individual pens that Risa could buy = 1
Amount that Risa needed to buy 42 pens
= 4 x 7 + 1 x 4 + 2
= 28 + 4 + 2
= $34
Amount that Risa needed to save more
= 34 - 27
= $7
Answer(s): (a) 33; (b) $7
