16 by 8 Keyboard matrix I am working on a project, trying to construct an 80 button keypad. My problem is how to multiplex it so that i only feed the arduino with two sets of BCD or binary(whichever is easier) which are supposed to drive a 75 led display board simultaneously with a pair of 7 segments. The led display board is remote, which means i should drive the boards serially. The other 5 buttons remaining out of 80 are for other functions such as clear last number etc.
Please help!
Easiest would be to use 2 arduino's, which are connected in some way.
One of them should tell the other one what to do. I suggest you make the one reading the buttons the "master" and the other one it's "slave" device.

You could use 8 inputs, for each column and 10 outputs for each row.
Then you check all 8 inputs per column (pretty rapidly) and then you'll be able to read every button.

You could choose to either send changes or just every value to the slave device.

The slave device would have to do something similiar, but with 2 outputs, to address the right LED's/segments.

Alternatively you could use shift-registers.

There actually are a lot of tutorials on this to be found on the web.
You should check out "Arduino multiplexing" or "Arduino led cube" tutorials.