C and D are the Roman numerals corresponding to 100 and 500 respectively. Hence, 5C = D. But, the larger point was that took 500 (5C) rather than 5000 (5K) yea

C and D are the Roman numerals corresponding to 100 and 500 respectively. Hence, 5C = D. But, the larger point was that took 500 (5C) rather than 5000 (5K) years to prove Fermat's Last Theorem.