When is 5 plus 4 equal 1001?

Answer 1

You could not really write it that way. Five plus four is nine, in the decimal system is 9, but in binary (base 2) it is 10012

So to be proper, you would write 100 + 101 = 1001 (where it is designated that you're using base 2 for the whole expression).

The binary system only has two numbers 1 and 0. Each place value is two times the one to the right (while in decimal, each place value is ten times the one to the right of it)

Or I guess you could write 510 + 410 = 10012 [10012=(1*8) + (0*4) + (0*2) + (1*1) = 9 base ten] Note: you should never say one thousand one for 1001 binary. You would say one zero zero one, or maybe One Oh Oh One.