In this section, I'll discuss word problems which give linear equations to solve. The difficult part of solving word problems is translating the words into equations. How can you learn to do this? When you're learning a foreign language, it's good to become familiar with lots of different words; with word problems, it's good to work with lots of different problems. I'll work through a variety of problems below.

* Example.* 68 less than 5 times a number is
equal to the number. Find the number.

Let x be the number. Note that "68 less than 5 times the
number" translates to the expression , *not* . So the problem statement gives

* Example.* When 142 is added to a number, the
result is 64 more than 3 times the number. Find the number.

Let x be the number. The problem statement gives

* Example.* Calvin Butterball buys a book for
$14.70, which is a discount off the
regular price. What is the regular price of the book?

Let x be the regular price. A discount is , so the discounted price is . Set this equal to 14.7 and solve for x:

If you travel for 2 hours at an average speed of 60 miles per hour, how far did you travel?

Your experience with travelling tells you how to figure this out:

That is,

Notice that you don't have to rely on just your memory to recall this
formula. You can figure out *what the formula should be* by
just asking yourself what you'd do in a simple case that is familiar
from real-life.

I will use this formula in the problems below.

* Example.* Two planes, which are 2400 miles
apart, fly toward each other. Their speeds differ by 60 miles per
hour. They pass each other after 5 hours. Find their speeds.

Since the planes started 2400 miles apart, when they pass each other
they must have *combined* to cover the 2400 miles.

- In problems involving distances, speeds, and times, draw pictures to help you see the what is going on.

So the sum of their distances is equal to 240:

One plane's speed is 210 miles per hour. The other plane's speed is miles per hour.

* Example.* Phoebe spends 2 hours training for
an upcoming race. She runs full speed at 8 miles per hour for the
race distance; then she walks back to her starting point at 2 miles
per hour. How long does she spend walking? How long does she spend
running?

Let x be the time she spent running. Since she spent 2 hours all together, she must have spent hours walking.

Since she ran out, then turned around and walked back, her running and walking distances must be equal.

Set the distances equal and solve for x:

She spends 0.4 hours running and hours walking.

* Example.* $1400 is divided between two
accounts. One account pays interest, while the other pays interest. At the end of one interest
period, the interest earned was $50. How much was invested in each
account?

Let x be the amount invested at . Since a total of $1400 was invested, must have been invested at .

600 was invested at and was invested at .

* Example.* After one interest period, the
interest earned on a $7000 investment exceeds the interest earned on
a $5000 investment by $160. The interest rate for the $5000
investment is greater than the
interest rate for the $5000 investment. Find the interest rates for
the two investments.

Let x be the interest rate for the $7000 investment. Then the interest rate for the $5000 investment is .

The interest earned on the $7000 investment is , and the interest earned on the $5000 investment is . The interest earned on a $7000 investment exceeds the interest earned on a $5000 investment by $160, so

The interest rates were for the $7000 investment and for the $5000 investment.

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Copyright 2013 by Bruce Ikenaga