To convert a binary number to its decimal equivalent follow these steps :
24 * 1 + 23 * 0 + 22 *1 + 21 * 0 + 20 * 1 = 21.
Therefore, the answer is 21.

 The maximum value in any number system is one less than the value of the base. The base in an octal number system is 8, therefore, the maximum value of the single digit is 7. It takes digits from 0 to 7.

 To convert an octal number to decimal number:
81 * 2 + 80 * 2 = 16 + 2 = 18.
Hence, the decimal equivalent is 18.

To obtain the octal equivalent, we take numbers in groups of 3, from right to left as :
 000  010  010   100

  0    2    2      4     =  (224)₈.

 Octal subtraction is done as follows:
417
– 232
________
165
The octal subtraction is the same as that of any other number system. The only difference is, like in a decimal number system, we borrow a group of 10, in a binary system we borrow a group of 2, in an octal number system, we borrow in groups of 8.