To determine the scale of a 20 cm model airplane, you need to know the actual length of the real airplane. Here’s how you can calculate the scale:

Formula:
Scale=Model LengthActual Airplane Lengthtext{Scale} = frac{text{Model Length}}{text{Actual Airplane Length}}Scale=Actual Airplane LengthModel Length​
Steps:

Measure or know the real airplane’s length in the same unit as the model (convert meters to centimeters if needed).
Divide the model’s length (20 cm) by the real airplane’s length.

Example Calculation:

If the real airplane is 40 meters (4,000 cm) long:

Scale=Model Length (20 cm)Real Airplane Length (4,000 cm)=204000=1:200text{Scale} = frac{text{Model Length (20 cm)}}{text{Real Airplane Length (4,000 cm)}} = frac{20}{4000} = 1:200Scale=Real Airplane Length (4,000 cm)Model Length (20 cm)​=400020​=1:200

So, the scale of the model is 1:200.

If You Know the Scale
If the model is labeled with a scale (e.g., 1:200), you can find the actual airplane’s length:

Actual Length=Model Length×Scale Denominatortext{Actual Length} = text{Model Length} times text{Scale Denominator}Actual Length=Model Length×Scale Denominator

For a 1:200 scale:

Actual Length=20 cm×200=4,000 cm=40 meterstext{Actual Length} = 20 , text{cm} times 200 = 4,000 , text{cm} = 40 , text{meters}Actual Length=20cm×200=4,000cm=40meters