Is it possible to add a multiplication and addition function for packed numbers when 1 byte contains 2 digits at once?
In the picture you can see the representation of the number 1234 in this form.

The code for converting two-digit decimal numbers to packed BCD and back.

char BcdToDecimal(char bcd) {
	return (char)((((bcd >> 4) & 0x0F) * 10) + (bcd & 0x0F));
}

char DecimalToBcd(char decimal) {
	return (char)((decimal / 10) * 16) + (decimal % 10);
}