In January 2021 it was determined that 200 persons (Police, Defense force & Prison officers) worked at least 40 hours of overtime each. Total overtime paid for these officers was $83,900 (each police officer received $500, each defense force officer received $400 and each prison officer received $300). If the sum of the defense force and prison officers exceeds the police officers by 18, determine how many defense force officers worked 40 hours of overtime in January 2021.