Simple algebra. Plug one of them into the other and simplify:

mv = code * 5000/1024

temp = (mv - 500) / 10

temp*10 = mv - 500

temp*10 = code * 5000/1024 - 500

temp*10 = (code * 5000) / 1024 - 500

temp*10 = ((code * 5000) >> 10) - 500

One multiply, one shift right by 10, and one subtraction. Then convert to BCD and display it.

However, if you have an 8-bit ADC, you're probably going to want to divide by 256 instead of 1024 (shift right by 8 instead of 10).

Simple algebra. Plug one of them into the other and simplify:

mv = code * 5000/1024

temp = (mv - 500) / 10

temp*10 = mv - 500

temp*10 = code * 5000/1024 - 500

temp*10 = (code * 5000) / 1024 - 500

(add divisor/2 before performing an integer division for rounding)

temp*10 = (code * 5000 + 512) / 1024 - 500

temp*10 = ((code * 5000 + 512) >> 10) - 500

One multiply, one shift right by 10, one addition, and one subtraction. Then convert to BCD and display it.

However, if you have an 8-bit ADC, you're probably going to want to divide by 256 instead of 1024 (shift right by 8 instead of 10) and add 128 instead of 512.

And here is an algorithm for converting binary to BCD: http://www.eng.utah.edu/~nmcdonal/Tutorials/BCDTutorial/BCDConversion.html

Just run that, split into groups of 4 bits, and drive each digit in the display with one group (after decoding to the proper 7 segment signals, of course).