The correct answer is:
125,000.
Explanation:
For the first turn, there are 50 numbers to choose from.
On each turn of a lock, the numbers can be repeated; this means on the second turn, there are 50 numbers to choose from, and on the third turn, there are 50 numbers to choose from. This gives us:
50³ = 50×50×50 = 125,000.
