If the primary gear ratio in a drill is 4: 1 and secondary gear ratio is also 4: 1 what is the overall gear ratio of the drill

I believe the answer would be 16:1
This can be looked at by assuming the motor speed is say 2000 RPM. Through the first reduction it is reduced 4 times to 500 RPM. The second reduction of 4:1 reduces it down to 125 RPM. The total reduction is the motor speed of 2000 RPM divided by the final speed of 125 or 16. A gear ratio is the ratio used to determine the angular speed and torque of a geared system. 

When you are making a guess you are forming a hypothesis