Answer: 5 hours

Balancing the speeds
The average speed is 60 miles per hour.

	speed	relative to average
first hour	45	60$-$15
second hour	45	60$-$15
third hour	70	60 + 10
etc.	70	60 + 10

After 2 hours, Mary is 30 behind an average of 60 mph
Each hour, she makes up 10 miles
She will need to do this for 3 hours


Using algebra
Let the time for which Mary drove at 70 mph be $t$ hours.
Then the total distance covered was $45 \times 2 + 70 \times t$ miles.

As her average speed over $2+t$ hours was $60$ mph, the total distance travelled was $60(2+t)$ miles.
Therefore $90 +70t = 120 + 60 t$, that is $10t = 30$, that is $t=3$.
So in total Mary's journey took $5$ hours.