I did this with a pencil and no calculator while my daughter was practicing her driving. Please let me know if you have trouble reading it. You'll have to add the calculator stuff yourself.
Both of the other angles are 40° because a triangle has a total angle of 180° and an isosceles triangle means that at least two of the sides have to be the same length (two angles must be the same) 180 - 100= 80 80 ÷ 2 = 40