How do I work out 100% if I know 5%?

OK I am having a really dumb moment. I am arguing with O2 (AGAIN) about discount on a new phone I am entitled too based on my top ups over the last 18months, now they say it is 5% of my top ups which they work out to be £16.89.

So to work out what 100% of my top ups are do I just do 2 x £16.89 = 33.78 (10%) then multiply that by 10 to give 100% eg £337.80 in 18 months then divide by 12 to get average monthly top up of £18.77.

Is that the right way to work it out? ...as I top up a lot more that £18 a month

Multiply 5% by 20 = 100%

So is that right...sorry I know I am being really thick it's justthat seems to be too straightforward
5% = £16.87
so multiply that by 20 =
£337.40

Sorry to be so stupid

5%=£16.89
100%=£337.80

I dont have a calculator but that is what I make it

Thank you!
Thats what I got it to be too!! Off to argue with O2 again!!!