David, Lisa, and Sam are working on a group project at school. David brings a stack of 20 index cards and places them on the table. Lisa takes 5 index cards to write some notes and then leaves the room for a moment. While she's gone, Sam accidentally spills water on the remaining index cards, making them unusable. He quickly goes to buy more index cards and returns with a new pack of 30 cards. When Lisa comes back, what will each group member think about the number of usable index cards? Respond briefly.
David initially brought 20 index cards to the table. After Lisa took 5 cards, there were 15 left. These cards were then ruined by Sam when he spilled water on them. Sam replaced the ruined cards by buying a new pack of 30 index cards. 

Now, considering each group member's perspective based on the information they have:

- David saw his original stack of 20 index cards being reduced to 15 after Lisa took some. He knows that Sam spilled water on the rest but also knows that Sam brought back new cards. Assuming he saw the new pack, David will think there are 30 usable index cards because he knows the original ones were damaged and replaced.

- Lisa only knows that she took 5 cards out of the original 20, leaving 15 behind. She left the room without knowledge of the accident, so assuming she didn't see the new pack or the damaged cards when she returns, Lisa would expect there to be 15 usable index cards left from the original stack she worked with.

- Sam knows that he made the original 15 remaining cards unusable with the spill. After buying the new pack, he is aware that there's a new set of 30 index cards available for use, replacing the ones that were ruined. Sam will think there are 30 usable index cards because he bought them himself and knows the previous stack was damaged.