There are 40 kids riding on a bus. 60% of the students have green backpacks. How many students have green backpacks? To solve this problem you need to know that the 40 kids represents the 100%, since they are all the kids on the bus. The kids with green backpacks is the "part" of the bus, which is 60%. 
There are many ways to solve this problem. Here are a few of the ways in no particular order. Make sure you understand at least a few of the methods.
Method 1 Since $60\mbox{%}=\frac{3}{5}$, you can find $\frac{3}{5}\,\mbox{ of 40}$. 
Method 2 40 students can be sorted into 10 groups of 4 students. Each group counts as 10%, so 60% equals 4 x 6. 
Method 3 60% can be written as the fraction $\frac{3}{5}$ and $\frac{3}{5}=\frac{24}{40}. So, $60\mbox{%}=\frac{60}{100}=\frac{3}{5}=\frac{24}{40}$ 
Method 4 Put the information in a ratio table and manipulate it to find 60%. 
Method 5 Use an equation to find the answer. The word "of" means "multiply". 
Method 6 Use a ratio table to set up a proportion. Then crossmultiply. 
Notice that all methods lead to the same answer: 24 students have green backpacks.
Example 2 In a room of 300 people, 12% of them are lefthanded. How many people is this? 
The important thing to know about this problem is the 300 people represent the 100%. We are to find the number of people that 12% represents. 
There are many ways to solve this problem. Here are a few of the ways in no particular order. Make sure you understand at least a few of the methods.
Method 1 Make a table and find that you can multiply the left column by 3 to get the right column. 
Method 2 Make a table and manipulate the rows until you get 12% in the left column, resulting in 36 in the right column. 
Method 3
Use an equation to find the part. The word "of" translates to "multiply". 
Method 4
Create a proportion using $\frac{part}{whole}=\frac{part}{whole}$. 
Method 5 12% is turned into a fraction and then the fraction is renamed to have a denominator of 300. 
Question 1 There are 35 people in a room. 20% of those people are lefthanded. How many people are righthanded?

[show answer] 
Question 2 James has 150 stamps. 45% of them are in excellent condition. How many stamps is this?

[show answer] 
Question 3 How much is 8% sales tax for a bicycle that costs \$240?

[show answer] 