A rectangle has side lengths \(2 \frac{1}{2}\,\text{cm}\) and \(3 \frac{1}{5}\,\text{cm}\). Jordan multiplies the whole-number parts and fractional parts separately: \(2\times3=6\) and \(\frac{1}{2}\times\frac{1}{5}=\frac{1}{10}\). Jordan claims the area is \(6 \frac{1}{10}\,\text{cm}^2\).
a) Without finding the exact area, explain why Jordan's result must be too small.
b) Find the actual area by rewriting the mixed numbers as improper fractions.
Hints
- Imagine splitting the rectangle into four smaller rectangles by separating each side length into a whole-number part and a fractional part.
- How do you rewrite a mixed number as an improper fraction?
- Look for common factors to cancel before multiplying.
Solution
1. For a), Jordan counted only two of the four partial areas. Splitting the side lengths into whole-number and fractional parts creates two additional positive products, \(2\times\frac{1}{5}\) and \(\frac{1}{2}\times3\), so the claimed area is too small.
2. For b), rewrite the side lengths: \(2 \frac{1}{2}=\frac{5}{2}\) and \(3 \frac{1}{5}=\frac{16}{5}\).
3. Multiply and simplify: \(\frac{5}{2}\times\frac{16}{5}=\frac{16}{2}=8\).
4. The actual area is \(8\,\text{cm}^2\).
Answer
a) Jordan omitted the two partial areas formed by multiplying a whole-number part by a fractional part, so the result is too small.
b) \(8\,\text{cm}^2\)