Adam saves $3 in the 1st week and $6 in the 2nd week.
Each week, he saves $3 more than the week before.
- How much money does he save in the 9th week?
- In which week does he save $21?
(a)
Amount that Adam saves on Week 1: $3
Amount that Adam saves on Week 2: $6 = 3 + 3 = 3 + (3 x 1)
Amount that Adam saves on Week 3: $9 = 3 + 3 + 3 = 3 + (3 x 2)
Amount that Adam saves on a certain week = 3 +(3 x (Week number - 1))
Amount that Adam saves on Week 9
= 3 + 3 × (9 - 1)
= 3 + 3 × 8
= 3 + 24
= $27
(b)
3 + 3 x (Week number - 1) = 21
3 x (Week number - 3) = 21 - 3
3 x (Week number - 3) = 18
Week number - 1 = 18 ÷ 3
Week number - 1 = 6
Week number = 6 + 1
Week number = 7
Answer(s): (a) $27; (b) 7
