7_ The sides respectively opposite the 30, 60, 90 degree angles have
lengths proportional to 1, √3, 2. This comes from chopping an
equilateral triangle in half and using the Pythagorean theorem. Also,
it is common to denote the angles by α, β, γ=90, and the corresponding
opposite sides by a, b, c.
So, you have α=30, β=60, γ=90, and b=20. Now a/b = 1/√3, so a=b/√3 = 20/√3, and c/b=2/√3, so c=2b/√3 = 40/√3.
Summary: the remaining parts are β=60, a=20/√3, c=40/√3.