I can see that 4 x 20(score) =80 and "four score" is clearly the answer.
So the clue becomes: "Seven ducks: therefore four score, making 80"
If cricket is being referenced, two ducks for each cricket match implies four matches, but there is nothing to suggest the batsman makes a score in the fourth match unless it is "making 80", in which case I do not think it is a well phrased clue, unless I am missing something.....