The three angles of a triangle are in arithmetic progression and the greatest is twice the least. The angles are:

In three terms in arithmetic progression the middle one is the average, so it is 180°/3 = 60° and the angles are 60° − $d$, 60°, 60° + $d$. The condition 60° + $d=2$(60° − d) gives $3d=60$°, so $d=20$° and the angles are 40°, 60°, 80° — and indeed 80° = $2\times 40$°. Each of the other lists is also an arithmetic progression summing to 180°, but there the greatest is three times, one and two-thirds times, and five times the least respectively.