SSC JE Civil Engineering 2018 Set 6 Solved Paper

For determining the support reactions at A and B of a three-hinged arch, points B and C are joined and produced to intersect the load line at D and a line parallel to the load line through A at D'.
Distances AD,DD' and AD' when measured were 4 cm, 3 cm and 5 cm respectively. The angle between the reactions at A and B is