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Explanation:

Let x, y and z be the number of children who took 1 rides, 2 rides and 3 rides respectively.

Since z = 20 and y + z = 55, y = 35.

Then, total number of rides = x + 2y + 3z = 145
⇒ x + 2 × 35 + 3 × 20 = 145
⇒ x = 15

Number of children, who did not try any of the rides
= 85 – (x + y + z)
= 85 – (15 + 35 + 20) = 15

Hence, option (c).