(1):(400-240)/(200-120)=2(2):(4*400+240)/(200-120)=23(3): t=(n*400+160)/(200-120)=5n+2
(1)240/(200-120)=3(2)(9*400+240)/(200-120)=48(3)t=(n*400+240)/(200-120)=5n+3
240/(200-120)=3 (9*400+240)/(200-120)=48 t=(n*400+240)/(200-120)=5n+3