Method 1
Assume the fractions are equal for now.
21/100 = 1/5
21*5 = 100*1 .... cross multiplication
105 = 100
The two sides are different numbers, so the original fractions aren't equal. We can fix the error by replacing each equal sign with a "greater than" sign as shown below.
21/100 > 1/5
21*5 > 100*1 .... cross multiplication
105 > 100
Think of it like working backwards up the chain.
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Method 2
Use a calculator or long division to find the following decimal values
We see that the first result is slightly larger. We can think of it like saying 21 is larger than 20.
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Method 3
To compare fractions, we need the denominators to be the same.
Let's convert 1/5 into a fraction with a denominator of 100. To do so, we multiply top and bottom by 20
1/5 = (1*20)/(5*20) = 20/100
In short,
1/5 = 2/100
When comparing 21/100 with 1/5, we're really comparing 21/100 with 20/100. We see that 21/100 is larger than 20/100 since 21 > 20.
So that is why 21/100 is greater than 1/5.