If the altitude of an equilateral triangle is 10, what is the length of each side?

All the angles of an equilateral triangle measures 60 degrees. The altitude measuring 10 is a side of the right triangle opposite to the angle. The side of the equilateral triangle becomes the hypotenuse to the right triangle generated by drawing the altitude. To get the measure of the hypotenuse, multiply the altitude to the sine of 60 degrees. The operation gives an answer of 8.66. 

Thus, the length of each side of the equilateral triangle measures approximately 8.66 units.