Answer:
In four minutes Heather can ring up 52 maybe even 53 customers.
Explanation:
To find this answer you would have to find how many customers Heather can rind up in one minute. To do this you can do 121 divided by 9 which equals about 13 so you can use 13. And then you would do 13 times 4 because that's how many minutes the question is asking for, and your answer would be about 52, so your answer is about 52 close to 53, you might not get an exact answer for a problem like this. :)