An old set contains these ten balance-scale weights: \(1\,\text{g}\), \(2\,\text{g}\), \(2\,\text{g}\), \(5\,\text{g}\), \(10\,\text{g}\), \(20\,\text{g}\), \(50\,\text{g}\), \(100\,\text{g}\), \(200\,\text{g}\), and \(500\,\text{g}\).
a) Which weights can be used to measure exactly \(387\,\text{g}\)?
b) Lucas says, “If I put all ten weights on the scale, they will weigh exactly \(1\,\text{kg}\).” Is he correct? Justify your answer with a calculation.
Hints
- Break \(387\) into hundreds, tens, and ones to find suitable weights.
- Look closely at the ones digit of \(387\).
- For part b, add all the weights carefully.
- How many grams would the total need to equal \(1\,\text{kg}\)?
Solution
1. Decompose \(387\,\text{g}\) as \(300\,\text{g} + 80\,\text{g} + 7\,\text{g}\). Use \(200\,\text{g} + 100\,\text{g}\), \(50\,\text{g} + 20\,\text{g} + 10\,\text{g}\), and \(5\,\text{g} + 2\,\text{g}\).
2. Add all ten weights: \(1 + 2 + 2 + 5 + 10 + 20 + 50 + 100 + 200 + 500 = 890\), so the total is \(890\,\text{g}\).
3. Since \(1\,\text{kg} = 1000\,\text{g}\) and \(890\,\text{g} < 1000\,\text{g}\), Lucas is not correct. The set is \(110\,\text{g}\) short of \(1\,\text{kg}\).
Answer
a) Use \(200\,\text{g}\), \(100\,\text{g}\), \(50\,\text{g}\), \(20\,\text{g}\), \(10\,\text{g}\), \(5\,\text{g}\), and one \(2\,\text{g}\) weight.
b) No. All ten weights total \(890\,\text{g}\), which is \(110\,\text{g}\) less than \(1\,\text{kg}\).