Two quantities are said to be in direct proportion if as one gets bigger, the other gets bigger or if one gets smaller the other gets smaller.

For this type of question it doesn't matter whether the C or D scale is used for the first quantity.  As a general rule set the quantity with the smaller starting digits on the D scale.

Worked Example:

A car takes 4 h to travel 200 km.  If it travels at a constant speed, how far does it travel in:

(a) 1h?
(b) 3h?

Since 200 has the smaller starting digit, I have chosen to set it on the D scale.  This means that I will set the distance (km) on the D scale and I will read the time (h) from the C scale.

1. Line up C4 with D200.
   (4h is set above 200km)
2. Move the cursor to C10 (or the right index marked as C1) and read D50.
   (I read this as ... for 1h, the distance traveled is 50km.)
3. Move the cursor to C3 and read D150.
   (I read this as ... for 3h, the distance traveled is 150km.)

Now you need to do some thinking as to the size of the numbers.  For example, when we read the answers on the D scale, for one hour, the distance couldn't possibly be 5km or 500km.  One hour is a quarter of four hours and 5 is nowhere near one quarter of 200 and 500 is far too much.  The same applies to the distance traveled in three hours.  Some call this common sense, but if you learn to use the slide rule properly and practice your number skills, you will be able to write down the answers without any problems.

Now let's try some practice questions, by clicking here!