To find the ratio of the number of burgers without onions to the number of burgers with onions, you can set it up as follows:
- Number of burgers without onions: 20
- Number of burgers with onions: 72
The ratio can be expressed as:
\[ \text{Ratio} = \frac{\text{Number of burgers without onions}}{\text{Number of burgers with onions}} = \frac{20}{72} \]
Next, we can simplify this fraction. Both 20 and 72 can be divided by their greatest common divisor (GCD), which is 4.
\[ \frac{20 \div 4}{72 \div 4} = \frac{5}{18} \]
So, the simplified ratio of the number of burgers without onions to the number of burgers with onions is:
\[ \boxed{5:18} \]
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