I am not sure are you given the whole question or partial of it. The full question is given in the following link.
filestore.aqa.org.uk/subjects/AQA-PHY6T-Q11-W-JUN11-TEST.PDF
See Q2 and Q3.
Your table is incomplete.
In question 3 (b) (i) it stated that:
It can be shown that the error in the total count C, is given by
uncertainty in total count C = ± √ C
Using data from the table on page 4, calculate the uncertainty in the
smallest total count, C.
When you go to page 4, you will see a value 429 when time is 140 minutes. That is where the mysterious number comes from.
% uncertainty =
CΔC× 100%
uncertainty in C = ± √ C