Digital Circuit to Check for Majority
Configure 31 MOS transistors as switchable current sources which feed a 32nd which is configured as a current sink at 15 times the source currents. Then observe the voltage of the summing node.

EDIT - I should not play games. The title said "Digital", so it's digital we'll go. One configuration would be 







simulate this circuit &ndash; Schematic created using CircuitLab

This handles 15 inputs. Duplicate it and add a 4-bit full adder and Bob's your uncle. I haven't shown the last adder, but you should be able to figure it out for yourself. 

And, of course, if you build this with BJTs, as with TTL or ECL logic, there will be no MOS transistors used at all.