If you travel 95 km at a constant speed of 50km/hr, and then 2.5 hours at a speed of 115 km/hr, what is your average speed fot the entire trip

Distance = rate*time   d = r*t, where rate is average speed.
To get the average speed for the whole trip, you need the total number of hours you were traveling. They don't give you the hours traveled at 95 km, but you can get that by dividing 95km/50km/hr. The km's cancel out and you're left with hours. So total time = (95km/50km/hr) + 2.5 hrs.
To get total distance, divide 115km/hr / 2.5 hrs.

Now that you have total time, t, and total distance, d, you can rearrange d = rt to find r.