Finding the Complement

When a part of a right angle carries a letter, the rule gives an equation instead of a number. The parts still add up to 90°.

Here the parts are (x + 10)° and 35°, so $\left(x+10\right)+35 = 90$. Collect the numbers: $x+45 = 90$, so $x = 45$. Check by putting it back: the first part is 55°, and 55 + 35 is 90.

On the board this line comes from Adjacent angles add: the two parts fill the corner ABC, and the board writes the square as 90. Tap the parts, pick the rule, and solve the equation it writes.