If eight trash bags last for thirty days then twenty eight trash bags will last how many days?

8 trash bags ... 30 days
28 trash bags ... x days

We would like to know what is the value of the x. You can do it in the following way: 
8 * x = 30 * 28
Now, you have to divide both sides of the equation by 8:
8 * x = 30 * 28    /8
x = 30 * 28 / 8
x = 105 (days)

The correct result is 105 days.