Can somebody not as stupid with maths as me please help me out?

I have a UK account so my currency is in GBP.

I want to be able to work out how much my GBP would be worth in Euros without having to use an external website or be given the value of the GBP to a euro. I want to be able to use my EUR/GBP chart.

For example, Xe.com will say:

1 GBP = 1.16268 EUR

1 EUR = 0.860084 GBP (this is same value given on EUR/GBP chart)

What I want to know is how can you calculate the GBP worth just by looking at the EUR worth of (in this example) 0.860084

I originally thought it would be:

2

subtract 0.860084

Answer = conversion rate

but that gives close results (1.139916) but not the exact figure of 1.16268 which is the correct rate and at larger quantities makes a big difference.

So if I had £10,000 how do I convert this to Euros using only the value of the Euro of 0.860084?

your help and thoughts is greatly appreciated.