The width of the poster is 35 inches.
What is the ratio proportion?
Proportion is an equation that defines that the two given ratios are equivalent to each other. In other words, the proportion states the equality of the two fractions or the ratios.
A photo with a length of 3 inches and a width of 5 inches is enlarged to poster size.
The poster and the photo are similar. the length of the poster is 21 inches.
Let x be the width of the poster.
The width of the poster is given by;
[tex]\rm \dfrac{Length \ of \ photo}{Width \ of \ photo}=\rm \dfrac{Length \ of \ poster}{Width \ of \ poster}\\\dfrac{3}{5}=\dfrac{21}{x}\\\\3 \times x =21 \times 5\\\\3x=105\\\\x=\dfrac{105}{3}\\\\x=35[/tex]
Hence, the width of the poster is 35 inches.
Learn more about ratios here;
https://brainly.com/question/14109516
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