The way I worked this out was -
As per B,
In 15 minutes, the maximum speed is 50/4 miles and minimum speed is 35/4 miles.
So we have 2 speeds, and we can calculate the average speed by using the relation Average speed is the ration of twice the product of two speeds to the sum of two speeds.
Va = 2V1V1/V1+V2
Where am i going wrong - Oh oh....we can use this when the distance is constant! $#&$# I messed it up!
these are two extrem speeds. there could be any rate of speeds in between these two speeds. so we cannot average these speeds.
we could have averaged these two extream speeds, if each of the speed had maintained for half of the time interval time.....