Answer:
Abby need to spent 5 days on vacation to buy 20 postcards.
Explanation:
Given:
Number of Postcard brought by Abby in 2 days of vacation = 8
Number of days of vacation = 2
We need find Number of days to spend on vacation to buy 20 Postcards
2 days = 8 postcards
1 day = Number of Postcard in 1 day
By using Unitary method we get;
Number of Postcard in 1 day =

Now we know that in 1 day he bought 4 Postcards So we need to find the Number of days to buy 20 postcards.
By Again using Unitary method we get;
Number of days to buy 20 postcards =

Hence Abby need to spent 5 days on vacation to buy 20 postcards.