The factor tree of the denominator of 13/80 is shown. After how many decimal places does the decimal expansion of 13/80 terminate?

Class 10Real NumbersNumber Systems

Step-by-Step Solution

80 = 2^{4} \times 5

, only 2s and 5s, so it terminates.

\frac{13}{80} = \frac{13 \times 5 ^{3}}{2 ^{4} \times 5 ^{4}} = \frac{1625}{1 0 ^{4}} = 0.1625

: 4 places, the larger exponent.

Answer: C) 4

**Theorem**: A rational number

\frac{p}{q}

(in lowest terms) has a terminating decimal expansion if and only if

q

has the form **2ᵐ × 5ⁿ** (m, n ≥ 0).

**Examples**:
-

\frac{3}{8} : q = 8 = 2^{3}

→ terminates

. (\frac{3}{8} = 0.375.) - \frac{7}{40} : q = 40 = 2^{3} \times 5

→ terminates

. (\frac{7}{40} = 0.175.) - \frac{1}{3} : q = 3

→ does NOT terminate

. (\frac{1}{3} = 0.333

… recurring.)
-

\frac{4}{15} : q = 15 = 3 \times 5

→ does NOT terminate (because of the 3).

**Why**: any decimal expansion is a fraction with denominator 10ⁿ = 2ⁿ × 5ⁿ. To convert

\frac{p}{q}

to decimal form, you need

q

to ONLY have 2s and 5s as primes — otherwise you can't multiply numerator/denominator into a power of 10.

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