Sounds like a bit of a brain fart under pressure tbh, what answer did you give? :/
I don't know the subject but I got halfway through yr post when I worked out you could have just picked any weight and the equation would be demonstrated the same, hell if I was in doubt i'd just add 'This is based on a weight of...' I'd expect to lose 1 mark at most for not using the defined average.
I can sympathise, I wasted an hour of a genetics exam because I had a brain fart and forgot what headings a table should have, and for the sake of a mark or two it simply wasn't worth the amount of time it took from others questions.