Answer:
4 trips
Step-by-step explanation:
First, let's put all data in minutes.
One trip to granny's home takes 35m to go plus 35m to come back, plans 4m for waking from the parking to the apartment and some other 4m for going to get the car to come home. So the total time spent on one trip is:
35+35+4+4=78 minutes
The total time he counted was 5h12m. If we put this in minutes we need tu multiply the hours by 60 (as every hour has 60 minutes) and sum the 12 minutes:
5*60 + 12 = 300+12=312 minutes total.
If 312 is total time and 78 is the time for one trip, the ratio gives the amount of trips:
312/78=4
So, he made 4 trips, all of them took 78 minutes (you can verify it by summing 78 4 times).