1) Gathering the data:
Scooter price: $172
Sid has $ 18
$11 w
2) We can solve this, by setting an equation. Since the problem says that Sid starts with $18 and earns $11 each week. As we don't know the number of weeks we can write:
11w+18=172 Subtract 18 from both sides
11w = 172 -18
11w = 154
w=14
So, if Sid carries on earning money that way He'll need 14 weeks to buy that scooter.