A proper fraction has the form \(\frac{n}{140}\), where \(1 \le n < 140\). Find every positive whole number \(n\) for which the fraction has a terminating decimal form. How many such fractions are there?
Hints
- Factor \(140\).
- Identify the prime factor that must disappear from the denominator.
- Determine what divisibility condition this places on \(n\).
- Count the multiples in the allowed range.
Solution
1. Factor the denominator: \(140 = 2^2 \cdot 5 \cdot 7\).
2. After simplification, a terminating decimal may have only prime factors of \(2\) and \(5\) in the denominator.
3. Therefore, the factor \(7\) must cancel, so \(n\) must be a multiple of \(7\).
4. The multiples of \(7\) from \(1\) through \(139\) are \(7, 14, 21, \ldots, 133\).
5. Since \(133 \div 7 = 19\), there are \(19\) possible values of \(n\).
Answer
\(n \in \{7, 14, 21, 28, 35, 42, 49, 56, 63, 70, 77, 84, 91, 98, 105, 112, 119, 126, 133\}\). There are \(19\) such fractions.