If you sketch this out you have an isosceles triangle and the angles are 45º. 45º and 90º.

The hypotenuse is the distance from the 0830 position to the NDB, the two equal sides are (i) the distance travelled between 0830 and 0840 and (ii) the distance from the 0840 position to the NDB.

In 10 minutes at 240 kts the aircraft will travel 40 nm so this is also the distance from the 0830 position and the NDB.