Answer :
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
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
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