Which of the following numbers is exactly divisible by 3?
Answer: B
A number is divisible by 3 if the sum of its digits is a multiple of 3.
A) 2+3+4+5+6 = 20 (Not divisible by 3)
B) 5+4+3+2+1 = 15 (Divisible by 3, since 15 = 3 × 5)
C) 7+8+9+0+1 = 25 (Not divisible by 3)
D) 1+3+5+7+9 = 25 (Not divisible by 3)