Dear Khan,

In the calculation of how long the runway will have to be for the plane to take off, you mentioned that we use average velocity (0 + 78)/2 times the time 78sec, we get 3042m which is the answer.  However, if I use another way: 1+2+3+.....+78=3081m.  Why there is such discrepancy?  Shouldn't the two ways get the same answer?  If I calculate the average velocity using (1+78)/2, then times 78sec, I get 3081m.   So could you explain what's wrong?  Thanks!

Best regards,

Wang Chen