甲数是乙数、丙数、丁数三数之和的二分之一，乙数是甲数、丙数、丁数

260÷(1- (1/2)/(1+1/2)- (1/3)/(1+1/3)- (1/4)/(1+1/4)) =260÷(1-1/3-1/4-1/5) =260×60/13 =1200