Adam saves $5 in the 1st week and $9 in the 2nd week.
Each week, he saves $4 more than the week before.
- How much money does he save in the 5th week?
- In which week does he save $21?
(a)
Amount that Adam saves on Week 1: $5
Amount that Adam saves on Week 2: $9 = 5 + 4 = 5 + (4 x 1)
Amount that Adam saves on Week 3: $13 = 5 + 4 + 4 = 5 + (4 x 2)
Amount that Adam saves on a certain week = 5 +(4 x (Week number - 1))
Amount that Adam saves on Week 5
= 5 + 4 × (5 - 1)
= 5 + 4 × 4
= 5 + 16
= $21
(b)
5 + 4 x (Week number - 1) = 21
4 x (Week number - 5) = 21 - 5
4 x (Week number - 5) = 16
Week number - 1 = 16 ÷ 4
Week number - 1 = 4
Week number = 4 + 1
Week number = 5
Answer(s): (a) $21; (b) 5