Jack have to cut 11 boards.
Solution:
Given that , out of the 52 boards jack used to build the deck, 20% of them had to be cut.
We have to find how many boards did he have to cut
Now, we know that, he have to cut 20% of 52 boards

Here, 0.4 represents not fully cut board, but still that board is cut and treated as cut board.
Hence, jack have to cut 11 boards.