How could i convert One number system to another system?

this is the output:

Choose a number system below:

Press A if you want Decimal to convert to another number system.
Press B if you want Binary to convert to another number system.
Press C if you want Octal to convert to another number system.
Press D if you want Hex to convert to another number system.

//when you pressed A this will be the output.

Enter Decimal:
The decimal equiv in binary is ____.
The decimal equiv in octal is ____.
The decimal equiv in hex is ____.

//teach me how to do it... please!

God bless! :)