Calculating Futures Price

Hey,

It is 10 July. A UK company has a US$6.65m invoice to pay on 26 August. They are concerned that exchange rate fluctuations could increase the £ cost and, hence, seek to effectively fix the £ cost using exchange traded futures. The current spot rate is $1.71110/£1.

Research shows that £/$ futures, where the contract size is denominated in £, are available on the CME Europe exchange at the following prices:

September expiry – 1.71035
December expiry – 1.70865

QUERY –
The solution posted that the IRF Price on 26th August to be 1.65750 (see below) Can someone help me figure out how?

Outcome on 26 August:
On 26 August the following was true:
Spot rate – $1.65770/£1
September futures price – $1.65750/£1