Remember that all the interior angles of a triangle add up to 180°.
The picture tells us two of the angles,
which is 40° and 
which is also 40°.
Subtract the measurement of these two angles from 180° to find the measure of the third angle,
:
180° - 40° - 40° = 100°.
The measure of angle RST is 100°.
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Answer: 100°
The picture tells us two of the angles,
Subtract the measurement of these two angles from 180° to find the measure of the third angle,
180° - 40° - 40° = 100°.
The measure of angle RST is 100°.
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Answer: 100°