While driving on a highway fiona averages 60 miles per hour .in the city she averages 30miles per hour .on a trip to her grandmother house fiona drives 3 1/2 hours on the highway and 1 1/2 hours in the city ? what is fiona's average speed for the entire trip to her grandmothers house ?

Fiona drives 3 1/2 hours on highway = 3.5 hours
average 60 miles per hour on highway.
Then 3.5 * 60 =210 miles 

1 1/2 hours in the city = 1.5 hours
average 30 miles per hour in city
Then 1.5 *30 = 45 miles

So
Fiona drives 210 miles on highway and 45 miles in city.
Total of her trip to her grandmother's house equals: 
210 + 45 = 255 miles

She drives 3.5 on highway and 1.5 in the city, then total driving should be:
3.5 + 1.5 = 5 hours

Average speed for the entire trip: 

255 /5 = 51 miles