Answer
The total time is 5.75 hours and the average speed is 852 km/hr.
Work Step by Step
First, we find:
$t_1 = \frac{2100}{720} = 2.92 ~hours$
$t_2 = \frac{2800}{990} = 2.83 ~hours$
Next,
The total time is: $t_1+t_2 = 2.92 + 2.83 = 5.75 ~hours$
Therefore, the average speed is $\frac{4900 ~km}{5.75~hours} = 852~km/hr$
