Janice S asked:
Two jets leave Dulles at 2 pm. One travels north at 850 mph and the other south at 750 mph. At what time will they be 4000 miles apart?
Two jets leave Dulles at 2 pm. One travels north at 850 mph and the other south at 750 mph. At what time will they be 4000 miles apart?
The truth is, I don’t even know where to begin. I tried looking it up in google, but since I didn’t even know what type of problem this was, I didn’t know what to put into the search engine.
An explanation of how you did each step would be nice! Thanks in advance! 
about 2.5 hours. you add 750 and 850 together and then multiply it by 2.5 and you get 4000
remember distance = rate * time
plane 1 = 850 mph
plane 2 = 750 mph
these are both rates so we need the sum of them
time = x
disatnce = 4000
d=r*t
so 4000 = x(850 + 750)
solve for x and add that amount of time to 2 pm and you have the answer
edit:
like the answer above me says, x= 2.5 so around 4:30 pm they will be 4000 miles apart.
After 1 hour the jets are 1*850+1*750=1600 miles apart (as one is traveling directly opposite the other. They average 800mph difference (1600/1 hour_ per hour of flight time. Divide 4,000 by 1600 (average distance created each hour) to get 2.5 hours total time.
Check: 2.5*750 + 2.5* 850= 1875+2125=4,000.
2pm+ 2.5 hours = 4:30pm
Ok, half the battle in all word problems is knowing when you need to use which information. First, they say the jets leave at 2 pm. This is only important at the end, so ignore this for now. Then they tell you the speed and direction of each jet. The speed is important, as is the direction. Then they ask the question, When will they be 4000 miles apart?
First, to solve this problem, look at the direction each jet is going. One is going North, the other South. Since they are going opposite directions, this will be a much easier problem because we can just add the speeds together to find the speed at which they are separating, which is 1600 mph.
850mi/h + 750mi/h = 1600mi/h
Now, we can solve the problem. How long will it take something going 1600 mph to go 4000 miles?
4000mi / 1600mi/h = 2.5h
One step left, which is to add up the time. 2.5 hours after 2:00pm is 4:30 pm, which is when your two jets will be 4000 miles apart.