QUESTION

CSAT

Easy

Maths

Prelims 2019

The ratio of a two-digit natural number to a number formed by reversing its digits is 4:74 : 7. The number of such pairs is

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Explanation

Step 1: Let the two-digit number be represented as: Original number: 10x+y10x + y Reversed number: 10y+x10y + x Here, xx is the tens digit, and yy is the unit digit.

Step 2: According to the problem, the ratio of the original number to the reversed number is 4:7. This gives us the equation: (10x+y)(10y+x)=47\frac{(10x + y)}{(10y + x)} = \frac{4}{7}

Step 3: Cross-multiply to get rid of the denominators: 7(10x+y)=4(10y+x)7 \cdot (10x + y) = 4 \cdot (10y + x)

Step 4: Expand both sides of the equation: 70x+7y=40y+4x70x + 7y = 40y + 4x

Step 5: Move all terms involving xx to one side and all terms involving yy to the other side: 70x4x=40y7y70x - 4x = 40y - 7y 66x=33y66x = 33y

Step 6: Simplify the equation by dividing both sides by 33: 2x=y2x = y

Step 7: Since xx and yy are digits of a two-digit number, they must be integers between 0 and 9. From the equation 2x=y2x = y, the possible pairs (x,y)(x, y) are: (1,2)(1, 2) (2,4)(2, 4) (3,6)(3, 6) (4,8)(4, 8)

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