It's been a tough day today. I've taken an Digital Electronics mid-semester examination where we're asked to do two awfully confusing questions (well, at least for me, that is). I would recommend the reader to have a basic knowledge of logic circuits before reading, as the explanation below will be confusing to you unless you do so. The first one asked us to make a logic circuit design that combines adder & subtractor in one, selectable through an input pin (Sub). See the diagram below to see the implementation.
Follow up:
The principle is simple. The circuit is supposed to carry out its operation on two 3-bit numbers. If the Sub input pin is set to LOW (logic 0), the circuit will function as an adder. Conversely, when it is set to HIGH (logic 1), it will function as a subtractor. I've unable to finish the question by the last 15 minutes of the exam, because I forgot the arrangement of gates used to make a Full Adder with 2HA+1OR, so I moved on to the next question. I've used and lost 1 full hour of exam time just to finish this first question! This is due to me being nervous, resulting in poor time management, I guess! Hahaha
On to the second question. It asked us to make a 4 to 1 multiplexer using a set of fixed minterm elements. On the first try I made it using a 16 to 1 (stupid I know, coz it's not what the question is asking 
), then I made a second logic diagram using 4 to 1 mux. I cannot remember it's minterm function so I cannot give you any diagram here, but eventually finished it in the last 10 minutes of the exam.. 
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yow yow....no 1 aku bnr dk e,,,,
i doubt that....
mgo b bnr.....amin
i prefer practice rather than theoritical nonsense, haha..