Isosceles at Work

With a letter in a label, the isosceles rule gives an equation: two equal base angles and the apex add up to 180. Write the base angle twice, as 2 times its label.

Here the apex is 50° and a base angle is (x + 10)°. Both base angles carry that size, so $2\left(x+10\right)+50 = 180$. Take 50 away: $2\left(x+10\right) = 130$. Divide by 2: $x+10 = 65$, so $x = 55$.

On the board, tap the labelled angles and pick Isosceles triangle. The board doubles the base angle for you, because the ticks say the other one matches.