Complex sentence: The defence was reliant on the 68 Spitfires and Hurricanes; a ratio of one RAF fighter to four German fighters and bombers or one British aircraft to every two German fighters.
Split into simpler sentences: The defence was reliant on the 68 Spitfires and Hurricanes. The numerical standing meant a ratio of one RAF fighter to every four German fighters and bombers, or in other words, one British aircraft to every two German fighters.