Magic stars

A magic star is composed of straight line segments.
Every straight line segment has three points with three numbers.
The sum of the three numbers in each segment is equal to a same number.
So, in a five-pointed magic star you can see numbers from 0 to 14 and the the sum of three numbers in each segment always is 21.
Exercises on five-pointed magic star

In a seven-pointed magic star you can see numbers from 0 to 20 and the the sum of three numbers in each segment always is 30.
Exercises on seven-pointed magic star

These exercises are very good for increase your mental calculations.