Задача от Австралийско кенгуру 2004 год 5-6 клас

[dorint]

Още една задача от Австралийско кенгуру 2004 год 5-6 клас :
30. Four 10 c coins lie on a table as
shown. Keeping in contact with
the other three coins, the shaded
coin is rolled around the other
three coins until it returns to its
starting place. Through what
angle does the shaded coin turn,
on its axis, in rolling once around
the other three coins?
И аз зная отговора,но не мога да я реша,а пък не сме учили градуси.