I have some historical data in a s/sheet that shows a $USD amt and an $AUD amt

What I need to do is to have a formula that calculates what the CONVERSION RATE was on the day when that historical transaction took place

Some real data I am using is:

$USD 51.20
$AUD 55.58

Looking at a table of historical rates this shows that on the 29th May 2014, the answer was 1.07480

BUT

I would like a formula to calculate actual conversion rate knowing the two amounts above

Any assistance greatly appreciated

Thanks & Regards,

BazzaBit