Question

A and B are vertical angles. If A = 4x - 10 and B = 6x - 30, find mA.

Answer 1

If A and B are vertical angles, then A = B.  Thus, 4x - 10 = 6x - 30, and 10 = x.  Check:  Does 4(10) - 10 = 6(10) - 30?  Does 30 = 30?  YES.  So x = 10 is correct.  The measure of angle A is 30 degrees.

Answer 2

vertical angles
m<A = m<B
so
4x - 10 = 6x - 30
6x - 4x = -10 + 30
2x = 20
x = 10

m<A = 4x - 10
m<A = 4(10) - 10
m<A = 40 - 10
m<A = 30