DIRECT PROPORTIONALITY
We say that there is a direct proportionality between two magnitudes if an increase on one magnitude causes a proportional increase on the other and a decrease on the first quantity causes a proportional decrease on the second.
Example 1:
With 8 gallons of petrol my car travels 248 miles. How far will it travel with 11 gallons of petrol?
+ gallons ==> + miles
 |
8 --------------- 248
11 --------------- x
It will travel 341 miles.
INVERSE PROPORTIONALITY
We say that there is an inverse proportionality between two magnitudes if an increase on one magnitude causes a proportional decrease on the other and a decrease on the first quantity causes a proportional increase on the second.
In general:
In general:
For example, imagine that 18 men can do a job in 12 days. However, if you needed to finish the job in just 6 days (half), you would need twice men, so you multiply men by two and divide divide days by two.
+ men ==> - days
| MEN | 18 | 36 | 9 | 3 |
| DAYS | 12 | 6 | 24 | 72 |
Example: The groceries in a home of 4 members are enough for 30 days. If a guest comes and stays with them, how many days will the groceries last?
+ members ==> groceries for - days
+ members ==> groceries for - days
 |
| There will be groceries for 24 days. |
