The XXX BRDC International Trophy was a motor race for Formula One cars held on 19 March 1978 at the Silverstone Circuit, England. It was the 30th running of the International Trophy, and the last to non-Championship Formula One regulations. The race was held over 40 laps of the Silverstone circuit, for a total distance of around 189 km .
The length of the BRDC International Trophy race is based on how many years it has been held. 
It is definitely incorrect because the race was in its 30th year, but was 40 circuits long, so the two are unrelated. I think it was difficult for the system because of the indirect wording. 