If she travels at 40 miles per hour then after 1 hour she has travelled 40 miles. That leaves 20 miles which is half of 40 so she needs to drive for another half hour to get 60 miles. That gives you 1 1/2 hours or 90 minutes.
40 miles = 60 minutes
so 20 miles = 60 minutes / 2 = 30 minutes
60 miles = 30 + 30 + 30
Speed (mph) = distance(miles) / Time(hrs)
Time (Hrs) = Distance(miles) / Speed (mph)
Time(hrs) = 60 miles/ 40 mph
Time(hrs) = 1.5 hrs = 90 minutes.
If she is driving at 40 miles per hour, how far will she drive in 1 hour (or 60 minutes)?
How far would she drive in 2 hours (or 120 minutes)?
How far would she drive in half an hour ( or 30 minutes)?
How far would she drive in a quarter of an hour (or 15 minutes)?
Now have another look at the question!
If she is travelling at forty miles per hour, this means she will have gone forty miles in one hour. If the lake is sixty miles away, and she can go forty miles in the first hour, this means she will have twenty miles left. Twenty miles is half of fourty miles, which means she will drive twenty miles in a half-hour. The last twenty miles will take her a half-hour to drive. Therefore, it will take her an hour and a half to reach the lake, which is sixty miles away. This is a simple problem, and I didn't need a formula, but you can also use a formula. R x T = D, or rate times time equals distance. You know the rate is forty, and the distance is sixty, so plug them into the formula. 40 x T = 60. Divide both sides by forty to isolate the time. What you do to one side, you must do to the other. You will get T = 1.5.