Given that during a total solar eclipse the Sun is fully hidden by the Moon, calculate the radius of the Moon. (You may use the following: Solar radius 700,000 km, Sun-Earth distance 150,000,000 km, Moon-Earth distance 400,000 km)

Let's prepare a diagram with the Sun on the right, Moon in the middle and Earth on the left. Their centres are collinear. Moreover, draw right angled triangles such that the right angle opens from the centre of the Sun to it's perimeter, and similarly for the Moon. Mark the known distances and radii and use similar triangles: the larger triangle has vertices at (1) centre of the Earth; (2) centre of the Sun; (3) perimeter of the Sun, and similarly for the Moon, with (1') centre of the Earth; (2') centre of the Moon; (3') perimeter of the Moon. The two triangles are similar because all three angles are the same, which is best demonstrated with a diagram. The ratio of the Moon radius to the Moon's distance from the Earth is equal to the ratio of the Solar radius to the Sun's distance from the Earth. Reordering this, we can solve for the radius of the Moon: Moon-Earth distance * Solar radius/Sun-Earth distance, which equates to ~2000km.