Three of the angles of a quadrilateral are 120 48 and 92 what is the size of the forth angle

The sum of the measures of the angles in a quadrilateral is 360 degrees.
360 - (120 + 48 + 92) = 100 degrees

First of all, you should know that ALL angles in a quadrilateral, meaning a shape having four angles like kite, rhombus, square,rectangle etc.. add up to 360 degrees. 
Then, to find the missing angle, you have to add up all the known angles, and minus it by 360 (which is the total of all angles in a quadrilateral)
So, 120 + 48 + 92 = 260. 
360 - 260 = 100. < your forth angle.

Hope this helps

)because it's going in order of the math sign