So first you make two isosceles triangles, one is URS and the other is UST. First find the degrees of angle RSU which is (180-120)/2=30. Then you know that TSU is 70 and then you can find angle T is 180-70*2 which is 40.
Another method, which is faster is that in a quadrilateral, all angles make 180 degrees. 120+100+100 is 320 and 360-320 is 40.