Question 760558
To solve this problem you have to find out long Luisa was driving in both parts.
For her time in the city:
{{{(1hour/30cross(miles))*(10cross(miles)/1) = 0.333hours}}}
For her time on the highway:
{{{(1hour/50cross(miles))*(10cross(miles)/1) = 0.2hours}}}

So Louisa drove 20 miles in about 0.533 hours

{{{20miles/0.533hours = 37.5miles/hour}}}
Her average speed was 37.5mph.

** You may get more exact values by converting the hour to minutes