https://youtu.be/RO3cgW8x5CI

#### Proportion

A proportion states that two ratios are equal.

A proportion comparing two ratios can be expressed as:

a is to b as c is to d

a : b = c : d

a/b = c/d

You can solve for any term in a proportion: a, b, c or d.

##### Example

Find the value of x. 35/84 = x/24

#### Inversely Proportional

Two quantities are directly proportional if one equals the other multiplied by a constant, or y = cx where c is a constant.

The Examples above have all been directly proportional.

4/15 = 16/x is the same as 4 × 4/15 × 4 = 16/x

35/84 = x/24 is the same as 35/84 = 3.5x/3.5 × 24

When two quantities are directly proportional, both values increase or both values decrease.

Two quantities are inversely proportional if one decreases when the other increases.

One value is equal to a constant divided by the other, or

y = c/x

which can be rewritten xy = c.

*examples*

The unemployment rate is inversely proportional to economic growth. Unemployment goes up when economic growth goes down.

The interest rate a company pays is inversely proportional to its credit rating. The better the credit rating, the lower the interest rate.

##### Example

Roger drove 90 miles in 1.5 hours. How far would he go in 2.5 hours?

https://youtu.be/O-H15E2exX4