[tex]Formula:c_{n} =c_{1} +d(n-1)\\c_{1} =-1;\;\;\;d=5\\c_{15} =c_{1} +d(15-1)= > c_{15} =-1 +14d= > c_{15} =-1 +14*5=69.\\c_{16} =c_{1} +d(16-1)= > c_{16} =-1 +15d= > c_{16} =-1 +15*5=74.\\c_{17} =c_{1} +d(17-1)= > c_{17} =-1 +16d= > c_{17} =-1 +16*5=79.\\c_{18} =c_{1} +d(18-1)= > c_{18} =-1 +17d= > c_{18} =-1 +17*5=84.\\c_{19} =c_{1} +d(19-1)= > c_{19} =-1 +18d= > c_{19} =-1 +18*5=89.\\c_{20} =c_{1} +d(20-1)= > c_{20} =-1 +19d= > c_{20} =-1 +19*5=94.\\[/tex]