Grade 6 FAST Mathematics Practice Test

In any quadrilateral, the interior angles add up to 360 degrees. Add the three given angles: 120 + 70 + 100 = 290. The fourth angle must bring the total to 360, so 360 − 290 = 70 degrees.

So the fourth angle measures 70 degrees. If a different value were used, the total of all four angles wouldn’t reach 360 degrees (for example, 40 would give 330, 60 would give 350, and 100 would give 390).