(1) Let this angle be α, then the complementary angle of this angle is 90-α, and the complementary angle is 180-α, 90-α/ 180-α = 3: 7, and the calculated α is 22.5. Or more simply, (180-90)/(7-3) = α.
(2)(3) You only need to list the quantitative relations between ∠A and ∠B and ∠C, and then bring them into the formulas of ∠B and ∠C to work out.
(2)∠A=36,∠B= 18 (3)∠A=85,∠B=5,∠C=95