input: Write a function to convert a base 10 number to a base 2 number.
  assert base10_to_base2(72) == '1001000'
  assert base10_to_base2(255) == '11111111'
  assert base10_to_base2(5) == '101'
output: def base10_to_base2(n):
      return bin(n)[2:]