Question: how to calculate the number of combinations in a lock?
Answer: Answer: There are 9,999 different combinations in a 4-digit lock.
Explanation: The 4-digit number on a lock begins from 0000 and ends up to 9999 which are:
From 0000 to 0999 there are 999 combinations. ( ... 
From 1,000 to 9,999 there are 8,999 combinations (9,000)
And now we have 1,000+9,000=10,000 combinations total.