Answer :
On Saturday he will have 36 ducks and the pattern is multiplying by 2 for each number
The pattern is: Ducks always swim on that pond in powers of 2 .
Monday ........ 2⁰ = 1
Tuesday ........ 2¹ = 2
Wednesday ... 2² = 4
Thursday ....... 2³ = 8
So
Friday ........... 2⁴ = 16
Saturday ...... 2⁵ = 32 <=== the answer
AND ...
If they kept doing this for a month, then
on the 31st day there would be
2³⁰ = 1,073,741,824 (more than 1 billion) ducks out there !