Item: tModelExplicitVerbalRuleArithmetic3a

Stem:

The rates that Zack charges for baby-sitting are as follows:

-- $2.50 per hour for the first child and

-- an extra 75 cents per hour for each additional child.

Choose the expression that represents the number of dollars that Zack will collect per hour for babysitting n children.

Options:

Key:

• Observable: 1. Score: 0; Evidence Value: 0.
  • Student:You're on the right track, but it's not quite there. You added a flat 75 cents, but there's a 75 cent charge for each additional child. The correct expression is 2.50 + .75(n-1)
  • Teacher:Student added in the 75 cents only for a second child.
• Observable: 2. Score: 0; Evidence Value: 0.
  • Student:Close, but not quite there. You added 75 cents for all the children, but the charge is just for additional children. The correct expression is 2.50 + .75(n-1)
  • Teacher:Student added in the additional charge for the first child, but did everything else right.
• Observable: 3. Score: 1; Evidence Value: 1.
  • Student:Correct!
  • Teacher:Student got it right.
• Observable: 4. Score: 0; Evidence Value: 0.
  • Student:You're on the right track, but it's not quite there. You forgot the base amount (2.50), and the 75 cents won't apply to all n-1 children the way you have it written. Parentheses are important here. The correct expression is 2.50 + .75(n-1)
  • Teacher:Student did not use the base amount and needs parentheses around the n-1.
• Observable: 5. Score: 0; Evidence Value: 0.
  • Student:You're on the right track, but it's not quite there. The 75 cents won't apply to all n-1 children, the way you have it written. Parentheses are important here. The correct expression is 2.50 + .75(n-1)
  • Teacher:Parentheses need to go around the n-1.

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