Because the triangle ABC is Rt triangle, and the angle A=30 degrees, AB= 10M (known).

So BC =1/2ab = 5m (in RT triangle, the right-angled side opposite to the 30-degree angle is equal to half of the hypotenuse).

Angle B= 180 degrees -30 degrees -90 degrees =60 degrees (the sum of the internal angles of the triangle is equal to 180 degrees).

Because CB 1 is perpendicular to AB (known)

So the triangle BB 1C is the Rt triangle.

So the angle B 1CB= 180 degrees -90 degrees -60 degrees =30 degrees.

So b1b =1/2bc = 2.5m (in an RT triangle, the right-angled side facing a 30-degree angle is equal to half of the hypotenuse).

So ab1=10-2.5 = 7.5m.

Because B 1C 1 is perpendicular to AC.

So the triangle AB 1C 1 is the Rt triangle.

And because the angle A=30 degrees, ab1= 7.5m.

So B 1C 1

=1/2ab1= 3.75m (in RT triangle, the right angle side of 30 degrees is equal to half of the hypotenuse).