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Question

By what number should \(10\frac{2}{3}\) be divided to obtain 20?

This question was previously asked in
RRB ALP 2018 CBT 2 Fitter Question Paper (21-Jan-2019) (Shift 3)
The correct answer is

8/15

Solving the Mixed Number Division Problem

The problem asks us to find a number that, when used to divide the mixed number \(10\frac{2}{3}\), results in 20. We can represent this problem as an equation.

Let the unknown number be \(x\).

The equation can be written as:

\(10\frac{2}{3} \div x = 20\)

First, we need to convert the mixed number \(10\frac{2}{3}\) into an improper fraction. To do this, multiply the whole number part (10) by the denominator (3) and add the numerator (2). Keep the original denominator (3).

\(10\frac{2}{3} = \frac{10 \times 3 + 2}{3} = \frac{30 + 2}{3} = \frac{32}{3}\)

Now substitute this improper fraction back into our equation:

\(\frac{32}{3} \div x = 20\)

Dividing by \(x\) is the same as multiplying by the reciprocal of \(x\), which is \(\frac{1}{x}\).

\(\frac{32}{3} \times \frac{1}{x} = 20\)
\(\frac{32}{3x} = 20\)

To solve for \(x\), we can multiply both sides of the equation by \(3x\) to eliminate the denominator on the left side:

\(\frac{32}{3x} \times 3x = 20 \times 3x\)
\(32 = 60x\)

Now, isolate \(x\) by dividing both sides of the equation by 60:

\(x = \frac{32}{60}\)

The resulting fraction \(\frac{32}{60}\) can be simplified. Find the greatest common divisor (GCD) of 32 and 60. Both numbers are divisible by 4.

\(\text{GCD}(32, 60) = 4\)

Divide the numerator and the denominator by their GCD:

\(x = \frac{32 \div 4}{60 \div 4} = \frac{8}{15}\)

So, the number by which \(10\frac{2}{3}\) should be divided to obtain 20 is \(\frac{8}{15}\).

Let's check our answer:

\(10\frac{2}{3} \div \frac{8}{15} = \frac{32}{3} \div \frac{8}{15}\)

Dividing by a fraction is the same as multiplying by its reciprocal:

\(\frac{32}{3} \times \frac{15}{8}\)

We can cancel common factors. 32 and 8 have a common factor of 8. 3 and 15 have a common factor of 3.

\(\frac{\cancel{32}^4}{\cancel{3}^1} \times \frac{\cancel{15}^5}{\cancel{8}^1} = \frac{4}{1} \times \frac{5}{1} = 4 \times 5 = 20\)

The check confirms that our calculated value for \(x\) is correct.

Revision Table: Key Steps

Step Description Mathematical Operation
1 Represent the problem as an equation \(10\frac{2}{3} \div x = 20\)
2 Convert mixed number to improper fraction \(10\frac{2}{3} = \frac{32}{3}\)
3 Substitute into equation \(\frac{32}{3} \div x = 20\)
4 Rewrite division as multiplication \(\frac{32}{3} \times \frac{1}{x} = 20\)
5 Simplify the left side \(\frac{32}{3x} = 20\)
6 Solve for x \(x = \frac{32}{60}\)
7 Simplify the fraction \(x = \frac{8}{15}\)

Additional Information: Understanding Mixed Numbers and Division

A mixed number combines a whole number and a fraction, like \(10\frac{2}{3}\). To perform calculations easily, we often convert them to improper fractions, where the numerator is larger than the denominator. For \(10\frac{2}{3}\), this means \(\frac{32}{3}\).

Dividing by a number is the inverse operation of multiplying by that number. For example, dividing by 5 is the same as multiplying by \(\frac{1}{5}\). Similarly, dividing by a fraction, say \(\frac{a}{b}\), is equivalent to multiplying by its reciprocal, \(\frac{b}{a}\).

In this problem, we used the property that if \(A \div x = B\), then \(A = B \times x\), and therefore \(x = \frac{A}{B}\). We applied this with \(A = \frac{32}{3}\) and \(B = 20\), leading to \(x = \frac{32/3}{20}\). Calculating \(\frac{32/3}{20}\) is the same as \(\frac{32}{3} \div 20\), which is \(\frac{32}{3} \times \frac{1}{20} = \frac{32}{60} = \frac{8}{15}\). This provides an alternative way to solve the equation \(\frac{32}{3} \div x = 20\), by treating \(x\) as the divisor and rearranging the equation.

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