A plane travels 70 feet in 1/10 second . At the same speed,how many
feet will it travel in 3 1/2 seconds?

Hi jatl3000:

First, let's calculate the plane's speed in feet/sec. 

speed = distance/time
      = 70 ft/.1 sec
      = 700 ft/sec

Now, let's calculate the total distance traveled in 3.5 seconds:

distance = speed * time
         = 700 ft/sec * 3.5 sec
         = 2450 ft

So, the plane will travel 2450 feet in 3 1/2 seconds.

Hi:

You can solve this problem by asking yourself "how many 1/10 second
intervals are there during 3 1/2 second?  That is, how many times can
I count off 1/10 second during the entire 3 1/2 seconds"

You might not know off the top of your head, but it's not hard to
figure out.  That's the kind of problem that division was invented
for.  All you have to do is divide 3 1/2 by 1/10.  That tells you how
many 1/10s are in 3 1/2.

Do you know how to divide 3 1/2 by 1/10?  The answer is 35, which is
easy to see if you used a calculator, but a little bit harder to
figure out if you're not allowed to use one.  (Not much harder,
though!  You just have to learn a trick about decimal points.)

Anyway, now that you know that there will be 35 of these 1/10 second
intervals during 3 1/2 seconds, you can figure out how many feet the
plane moves.  Since it moves 70 feet in each 1/10 second interval, and
since there are 35 of those intervals, all you have to do is multiply
70 times 35.  It's the exact same as if someone asked you, say, how
many apples have you got in 70 baskets if each basket holds 35 apples.