What is the four-digit number in which the first digit is one-third the second, the third is the sum of the first two, and the last is three times the second?
Pronounced as 1 letter, And written with 3, 2 letters there are, and 2 only in me. I’m double, I’m single, I’m black blue and grey, I’m read from both ends, and the same either way. What am I?