23.2 mph
What you do is figure out how far the gondola is moving. Given that it's moving in a circle with a radius of 25 feet, the circumference is:
2 * 3.14159 * 25 = 157.08 feet.
Now it travels that distance 13 times per minute giving:
157.08 * 13 = 2042.04 feet per minute.
Finally, assuming it keeps up that speed for 1 hour, you get
2042.04 * 60 = 122522.4 feet per hour.
Now convert that to miles per hour by dividing by 5280
122522.4 / 5280 = 23.2
