Evaluating formulas

Solution to question 1 continued

The formula could be written as Child amount = \(\dfrac{\text{Adult Amount}\,\times\,\text{Age of Child}}{\text{Age of Child} + 12}\)

Child amount		= \(\dfrac{10\times 8}{8+12}\)
		= \(\dfrac{80}{20}\)
		= 4 mL.

If using a calculator and you type 10 × 8 ÷ 8 + 12, the answer will be 22 mL which is more than the adult amount. Brackets must be used: (10 × 8) ÷ (8 + 12).
The following is a common error.

Child amount = \(\dfrac{10\times8\hspace{-2.2mm}/}{8\hspace{-2.2mm}/+12}\) = \(\dfrac{10}{12}\) = \(\dfrac{5}{6}\).

You cannot cancel as the denominator involves addition, not multiplication.

Evaluating formulas

Solution to question 1 continued