A, B, C, D form a complete circle which is 360 degrees.
Add all 4 together:
A + 2A + 4A+ 3A = 360
10A = 360
Divide both sides by 10:
A = 360/10 = 36
Now we know A so can calculate the rest:
B = 2A = 2(36) = 72 degrees
C = 4A = 4(36) = 144 degrees
D = 3A = 3(36) = 108 degrees