Answer: Michelle spent $4.50 on candy last week.
Explanation: According to the problem, Michelle spent 4 times as much money on snacks this week as she did last week, so this week's amount would be 4x. And we know that Michelle spent $18 on candy this week, so we can write the following equation:
4x = 18
To solve for x, we need to divide both sides of the equation by 4:
x = 18 / 4
x = $4.50
So, Michelle spent $4.50 on candy last week.