A motorist makes a trip of 180 miles. For the first 90 miles she drives at a constant speed of 30 mph at what constant speed must she drive the remaining distance if her average speed for the total trip is to be 40 mph

Remaining Distance = 180miles-90miles = 90miles

Time taken for first part of her journey = 90miles / 30miles/h = 3hours

To have average speed of 40mph,

Let the time taken for second part be xh

180 / 3h+xh = 40

3+x = 4.5h

x = 1.5h

Constant speed she must drive = 90miles / 1.5h = 60mph