Triangles angles always add up to 180 degrees. Add the known degrees togther and subtract from 180 to get what the finall angle is. For example, if a triangle has two known angles, 50 degrees and 80 degrees add them and get 120 then do 180-120 to get the missing angle. hope that helps sorry if it's confusing, Just ask if i need to explain more :)
Answer: Sample response: 50% is a friendly percent. You can divide 120 by 2 to find the answer. 120 divided by 2 is 60. So, 60 students were 7 years old when they lost their first tooth.