Electronic – Finding number of don’t care conditions

"Don't Care" conditions still exist in your design, you are just saying that you don't care what happens when this input set occurs. When you map the resulting truth table and perform appropriate grouping (either 1s or 0s), you effectively establish what the don't care outputs will be.

Namely, if you group a don't care condition with 1s (minterms), it becomes a 1. This of course is also true for don't care conditions and grouping with 0s (max terms).

For example, your input C'B'A is now a 1 since you have grouped it as such. The point of this is to relax restrictions on the output for states that don't really matter to you so that you have increased freedom when mapping.

You must either establish what happens when this undesirable input occurs (i.e. turn the segment off by driving all LEDs off), or ensure that such input combinations never occur in your product as it will elicit undefined behavior.

I understand you want to find out which of the two numbers is larger? You can do that by comparing the bits of A and B, starting at the highest ones, let's call those A3 and B3.

There are three cases: they are equal, or otherwise A3 is 1 (and B3 is 0), or B3 is 1 (and A3 is zero). In the first case (equal), which one is larger is determined by the next (lower) bit. If they are not equal, take the A3 bit and it tells you whether A is larger. So for the left most bit:

A_is_larger = ( ( A3 == B3 ) * ( input from lower bits ) ) + ( ( A3 != B3 ) * A3 ).

For the rightmost bit the you can simplify this to A1 (assuming that for equal values you don't care what the outcome is).