Correct me if I'm wrong, but I think the answer is 7/10.
Tiffany's amount which is 2/5 is also equal to 4/10, so subtract 1/10 off of Tiffany's amount to get Linda's amount. That'll give you 3/10. Now it's asking how many they bought together so you'll add 3/10 and 4/10 which will give you 7/10.