A football player is running back and forth along the sideline of the football field. He starts at the 50 yard line, in the middle. He runs to the left all the way to the 20 yard line, then he runs to the right and stops at the 40 yard line on the other side of the field. What is the total distance travelled and what is his displacement?

20 yards East

Explanation:

The question states he starts at the 50 yard line, and didn't imply that he'd ran for 50 yards already. Effectively, that said 50 yard would not count.

The question proceeded to saying he ran leftwards for 20 yards. This is the distance, or displacement that we would use as the first measurement. Running 20 yards in the Eastern direction.

The question then proceeded again, to saying he ran rightwards for another 40 yard. By so doing, he eclipsed the initial 20 yards he'd ran. And added few more yards..

40 - 20 = 20, meaning, he ran 20 more yards in the rightward direction.

Therefore, his displacement would be 20 yards to his right or say, Eastern direction.

B. 100 cm
The object is located where the image is as well, so so=si. Substitute this into the equation sosi=f^2, and you'll get s^2o=f^2. This tells you that so is equal to f (focal length) when disregarding the negative. Therefore, the focal point has to be exactly halfway between the object (200cm) and the mirror (200cm).