Adam saves $3 in the 1st week and $9 in the 2nd week.
Each week, he saves $6 more than the week before.
- How much money does he save in the 7th week?
- In which week does he save $21?
(a)
Amount that Adam saves on Week 1: $3
Amount that Adam saves on Week 2: $9 = 3 + 6 = 3 + (6 x 1)
Amount that Adam saves on Week 3: $15 = 3 + 6 + 6 = 3 + (6 x 2)
Amount that Adam saves on a certain week = 3 +(6 x (Week number - 1))
Amount that Adam saves on Week 7
= 3 + 6 × (7 - 1)
= 3 + 6 × 6
= 3 + 36
= $39
(b)
3 + 6 x (Week number - 1) = 21
6 x (Week number - 3) = 21 - 3
6 x (Week number - 3) = 18
Week number - 1 = 18 ÷ 6
Week number - 1 = 3
Week number = 3 + 1
Week number = 4
Answer(s): (a) $39; (b) 4