Answer:
17 minutes
Explanation:
To answer this question, all we have to do is subtract the original amount of money on the phone card by what is remaining after Keiko's call:
20 - 17.28 = 2.72
Now, we divide this number by 0.16:
2.72 / 0.16 = 17
So, Keiko's call lasted 17 minutes
Hope this helps :)