The ratio of a two-digit natural number to a number formed by reversing its digits is 4:7. The number of such pairs is
Let the digit at unit's place be a.
Let the digit at ten's place be b.
Original number = 10b + a
Number formed on reversing the digits = 10a + b
According to the question,
10b + a : 10a + b = 4 : 7
70b + 7a = 40a + 4b
2b = a
When b = 1, a = 2; numbers are 12 and 21.
When b = 2, a = 4; numbers are 24 and 42.
When b = 3, a = 6; numbers are 36 and 63.
When b = 4, a = 8; numbers are 48 and 84.
When b = 5, number become three-digit number.
Therefore, only 4 pairs are possible.
The correct option is B.