Fiona finds1shell on her first day at the beach. Every day that week she finds twice as many as she did the day before. How many shells did Fiona find on the seventh day?

If she found 1 on Monday so day 11 (I'm using Monday to start with because it keeps track of the days)

That would be 2 on Tuesday so day 2

Then 4 Wednesday day 3

8 Thursday day 4

16 Friday day 5

32 Saturday day 6

And finally it would be 64 on Sunday aka the 7th day

64 because

she starts with 1

1x2 is 2

2x2 is 4

4x2 is 8

8x2 is 16

16x2 is 32

and 32x2 is 64