Question 459138
Note: I changed your question to read "RECTANGLE" rather than "TRIANGLE." I assume that you meant rectangle because the question is filed under RECTANGLES.

Notice that one given measurement is metric and the other is not. Let's convert 20 inches to centimeters. There are 2.5 centimeters in each inch. We want to know how many centimeters are in 20 inches, so we multiply: 
{{{20*2.5=50 cm}}}.
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Let
P = the perimeter of the rectangle = 50 cm
W = the width of the rectangle = unknown
L - the length of the rectangle = 13 cm
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Perimeter is defined as the distance around the sides of the rectangle. One formula for calculating the perimeter is {{{2W+2L=P}}}.

We will use the perimeter formula to find the width of the rectangle. Substitute the values we know for the perimeter and length into the formula. Use algebra to solve for W, the width.

{{{2W+2L=P}}}
{{{2W+2(13)=50}}}
{{{2W+26=50}}}
Subtract 26 from both sides.
{{{2W=50-26}}}
{{{2W=24}}}
Divide both sides by 2
{{{W=12}}}
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Using algebra, we have found that the width is 12 cm. Let's check our work.
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CHECK:
{{{2W+2L=P}}}
{{{2(12)+2(13)=50}}}
{{{24+26=50}}}
{{{50=50}}}  (Perfect!)

The length of the rectangle is 13 cm.
The width of the rectangle is 12 cm.

Hope this helps!