if 10001 is number 17, then number 18 is?

Answer 1

10001 is 17 in binary arithmetic, that is in artithmetic that uses only the symbols 0 and 1 to represent all the numbers.
To get 18 in binary arith. we want 1 + 10001. We can't put a
"2" at the end because we don't have 2's available.
Think of what you would do if you added 1 to 1009 in the decimal system. We don't put a "10" as the last digit because we don't have "10" as a separate symbol. We
exchange the 9 for a 0 and the 0 next to the 9 is increased by 1. Do the same with 1 + 10001. Change the rightmost 1 to a 0 and the 0 next to it to a 1 to get 10010.

Answer 2

This is binary (base 2)
64_32_16_8_4_2_1
_______1_0_0_0_1 = 17 base ten
_______1_0_0_1_0 = 18 base ten