Step-by-step explanation:
You want to know how to measure an angle using a protractor.
Protractor
A protractor is the tool used to measure angles. It will generally be made of transparent plastic, inscribed with scales in an arc that covers 180 degrees. The one shown in the attachment is typical, in that it has scales from 0 to 180° in both the clockwise and counterclockwise direction.
Method
The tool is placed on the angle being measured so that the center of the arc is on the vertex of the angle. Align one of the lines marked with 0 degrees with one ray of the angle. Where the other ray crosses the scale you're using, the measure of the angle can be read. The graduations are generally in units of 1 degree. The attachment shows an angle of 72°.
You can usually read the angle to the nearest degree. If you are very careful in your alignment, and the angle is drawn with fairly skinny lines, you may be able to interpolate the angle measure to a suitable fraction of a degree.
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Additional comment
The idea of "interpolation" may be a bit advanced for your 3rd-grade student.
Using a protractor is the most direct way to measure an angle. Other methods involve measuring legs of a triangle that includes the angle of interest, then doing calculations. That, too, may be a bit advanced for 3rd grade.
Numerous websites provide videos describing this process.
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