The decimal number 39 can be converted to binary (base 2) as follows:

1. 39 divided by 2 equals 19 remainder 1
2. 19 divided by 2 equals 9 remainder 1
3. 9 divided by 2 equals 4 remainder 1
4. 4 divided by 2 equals 2 remainder 0
5. 2 divided by 2 equals 1 remainder 0
6. Finally, 1 divided by 2 equals 0 remainder 1

Reading the remainders from bottom to top gives the binary representation of the decimal number. Therefore, the decimal number 39 is 100111 in base 2.