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