Question 65880
Let the angles be: A, B, and C where angle C is the smallest.
From the problem description, we can write:
A = 2C
B = C+20 
And, of course, in any plane triangle, the sum of the angles is 180 degrees, so...
A+B+C = 180  Substitute the A and B from above and solve for C, the smallest angle.
2C+C+20+C = 180 Simplify.
4C+20 = 180  Subtract 20 from both sides.
4C = 160  Finally, divide both sides by 4.
C = 40 degrees.

Check:
A+B+C = 2(40)+(40+20)+40 = 80+60+40 = 180 Good!