Binary for 31
If you lifted all of your fingers, that would be 11111. That's binary for the decimal number 31. (Hint: Add up all the numbers you wrote on your fingers that are showing...what do you get?)
Binary for 30
With these fingers lifted, you're showing 11110. If you add up the numbers written on your fingers, you get 30, which is the decimal number for 11110.
Binary for 6
If you lifted your fingers with 4 and 2 written on them, that would be 00110, which is the binary number for 6.
Binary for 3
If you lifted just your thumb and second finger, you'd be showing 00011, which is the binary number for 3.
Binary for 1
If you kept all fingers down except the finger with the decimal number 1 drawn on it, you'd be showing 00001, the binary number for 1.
Can you come up with the binary number for 12?
Hold up the finger with the number 8 and the finger with the number 4. You're actually holding up 12, right? This is because 8+4 = 12. Now look at your fingers. You should have 01100 because your second and third fingers are up, but the rest are down. That's the binary for the number 12!