Answer
$ 70^{\circ}$
Work Step by Step
In an isosceles triangle, the base angles ( angles opposite to equal sides) are also equal.
Assuming that each base angle measures $ x^{\circ}$
Now all the three angles of the said triangle are $ 40^{\circ}$ , $ x^{\circ}$ and $ x^{\circ}$
We know that all the three angles of a triangle add up to $ 180^{\circ}$,
Therefore
$ 40^{\circ}$ + $ x^{\circ}$ + $ x^{\circ}$ = $ 180^{\circ}$
$ 40^{\circ}$ + $ 2x^{\circ}$ = $ 180^{\circ}$
$ 2x^{\circ}$ = $ 180^{\circ}$ - $ 40^{\circ}$
$ 2x^{\circ}$ = $ 140^{\circ}$
$ x^{\circ}$ = $\frac{140}{2}$ = $ 70^{\circ}$
Thus measure of each base angle is $ 70^{\circ}$
