At what percentage per annum will a sum of money double in 8 years :

In the series one term is wrong. Find the wrong term :

(a)1/2,1, 3, 12, 60, 300, 2520

0.14 x 0.14 = ................

6.28 0.002 = ............

Insert the correct mathematical operation :

4.............. 2.............. 5 = 13

A car travels from A to B at 30 kmph and from B to A at 20 kmph. Average speed is :