According to the first scale, 2 red balls = 1 star.
According to the second scale, 2 triangles = 1 star
So,
1 triangle = 1 red ball
1 star = 2 red balls
1 red ball = 1 red ball
Therefore, 2 + 1 + 1 = 4 red balls
The last scale needs 4 red balls
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