## Interview Question – BCD Digit, Multiplied by 5

December 21, 2008

A while back, someone sent me the interview question I am about to describe, asking for help. I think it serves a very good example of observing patterns and not rushing into conclusions.
I will immediately post the answer after describing the problem. However, I urge you to try and solve it on your own and see what you came up with. On we go with the question…

Design a circuit with minimum logic that receives a single digit, coded BCD (4 wires) and as an output gives you the result multiplied by 5 – also BCD coded (8 wires).

So, I hope you got a solution ready at hand and you didn’t cheat ðŸ˜‰ .

Let’s first make some order and present the input and required outputs in a table (always a good habit).

Looking for some patterns we can see that we actually don’t need any logic at all to solve this problem!!

You will be amazed how many people get stuck with a certain solution and believe it is the minimal one. Especially when the outcome is one or two single gates. When you tell them it can be done with less, they will easily find the solution. IMHO there is nothing really clever or sophisticated about this problem, but it demonstrates beautifully how it is sometimes hard for us to escape our initial ideas and approaches about a problem.

Coming to think of it, this post was more about psychology and problem solving than digital design – please forgive…