What is 20 written in base 3?

http://www.unitconversion.org/unit_converter/numbers.html

20 written in base 3 is 110 in binary

20 from the decimal to ternary (base 3) is 202.

First position is multiples of 9, then multiples of 3, followed by single units of 0, 1, or 2. Therefore, you need 2 multiples of 9 plus 2 single units.

https://en.wikipedia.org/wiki/Base_3

20 in binary is 10100.

The decimal (base 10) 6 is 110 in binary (base 2) and 20 ternary (base 3).

If you are actually intending to ask what is the decimal evuivalent of the base 3 number written as 20, then the answer is 6. As explained in the Base 3 link above, the 0 means 0x1 (=0), and the 2 means 2x3 (=6). 0 + 6 = 6

Right, the question is incomplete. If you want conversion to base 3, you have to state what base the number 20 is written in. If the phrase '20 written in base 3' means just what it says, what is the rest of the question?

For those who don't know what number bases mean at all, here is a lesson:
http://www.youtube.com/watch?v=3yGSA-7RP08

If the base of a number is unspecified, base 10 is usually assumed.
So, as it stands, the most likely meaning of the question is how does twenty (20 in base 10) appear when written in base 3?
The highest power of three less than twenty is 9 and there are two nines so the first digit is 2 in the nines column. This leaves a remainder of two (twenty minus eighteen) 0 in the threes column and 2 in the units column.
So 20 is written as 202 in base 3.
http://www.mathsisfun.com/numbers/convert-base.php