The measures of two angles of pairs of triangles are given. Which pairs of triangles are similar?

If the triangles are similar then the angles in both are equal. Let's look at each set individually:

(1) Triangle 1: 25°, 35°
Triangle 2: 25°, 120°
Now it may be hard to tell if the triangles are similar at the moment so we must calculate the third angle in each triangle (The angles in a triangle add up to 180°, therefor the missing angle = 180 - (given angle 1 + given angle 2)
Triangle 1: 180 - (25 + 35) = 120°
Triangle 2: 180 - (25 + 120) = 35°

Now writing out the set of angles again we have:
Triangle 1: 25°, 35°, 120°
Triangle 2: 25°, 120°, 35°

So in fact Triangle 1 and 2 are similar.

Now we can repeat this process for (2) - (5):

(2) Triangle 1: 100°, 60°, 20°
Triangle 2: 100°, 20°, 60°
This pair is also similar

(3) Triangle 1: 90°, 45°, 45°
Triangle 2: 45°, 40°, 95°
This pair is not similar

(4) Triangle 1: 37°, 63°, 80°
Triangle 2: 63°, 107°, 10°
This pair is not similar

(5) Triangle 1: 90°, 20°, 70°
Triangle 2: 20°, 90°, 70°
This pair is similar

Therefor pairs (1), (2) and (5) are similar