But the answer is 3.5 ?

Hello Bryan, I have double checked my workings again and they are correct.

We need to find the rate of Anna and Betty working together, and that gives us 1.6 stamps per day.

Then we need to find the individual rate. We can do that by making use of the information given when either one is absent. And the summation of their individual rates will be equal to 1.6 stamps per day.

I do not know where you got your answer from. It could be a typo.

okay, thank you :)

U r right. Answer is 4.5