CHECK BOX HELP PLEASE.

My sheet contains 80 rows. (80 competitors). Their results are posted, one
row to a competitor. Starting on row 7 - 87

I have a check box (default €“ unchecked) in cell K7. Its purpose is to
indicate, whether the competitors times qualify for the event, There are
three timers. The three times are averaged to give €œThe Time€�. If they run
over their time, this checkbox should be clicked and the previously entered
times in C7,E7,E8 should be replaced with NIL.
This check box will be in Col k on every row.
I can record a macro to do this on that line, but I want to use a relative
function so that I can copy it down the page to make give the same code to
every row, instead of having 80 macros. I have nearly been there but
everytime I get close, I seem to come up with something not quite right. Any
help very much appreciated.