In 1963, the united states postal service started using zip (zone improvement plan) codes. each zip code is a sequence of five digits. how many different u. s. zip codes are possible?

Here is how you find the number of possible U.S Zip codes. 
Take note of this: There are 10 1 digit numbers: 0,1,2,3,4,5,6,7,8,9.
And we have 5 slots. So in each slot, there can be 10 possible numbers. 
So let's put 10 in each blank slot. 
10, 10, 10, 10, 10.
Therefore, the final answer would be 100,000. There are 100,000 possibilities of different U.S zip codes. Hope this answer helps.