Since the triangle is rectangular, we can apply the Pythagorean theorem:
AB2 = BC2 + AC2
AB ^ 2 = 15 ^ 2 + 20 ^ 2
AB ^ 2 = 225 + 400 = 625
AB = 25
The smaller angle lies opposite the smaller side, 15 <20, therefore the sine of the smaller angle will be equal to the ratio of the smaller side to the hypotenuse, i.e. 15/25 = 3/5 = 0.6
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