What is the small positive two digit no. divisible by 3 and such that the sum of its digits is 9?  Choose one
A) 10 B) 12 C) 14 D) 18 E) 21
Let xy be the whole number with x and y the two digits that make up the number. The number is divisible by 3 may be written as follows   10 x + y = 3 k   The sum of x and y is equal to 9.   x + y = 9   Solve the above equation for y   y = 9 - x Substitute y = 9 - x in the equation 10 x + y = 3 k to obtain.   10 x + 9 - x = 3 k   Solve for x   x = (k - 3) / 3   x is a positive integer smaller than 10   Let k = 1, 2, 3, ... and select the first value that gives x as an integer. k = 6 gives x = 1   Find y using the equation y = 9 - x = 8   The number we are looking for is 18 and the answer is D. It is divisible by 3 and the sum of its digits is equal to 9 and it is the smallest and positive whole number with such properties.
<result>D</result>
What is the small positive two digit no. divisible by 3 and such that the sum of its digits is 9?