{T=N[ceil[Ln[1 - c]/Ln[ ((1 - a/((h - 20) (s - 11)))^(ceil( h*v*3*10^-5)))*((1 - b/((h - 20) (v - 19 - s)))^(ceil(h*v*1.5*10^-5))) ]]/3]} with h= 4200 ; v= 1200 ; s= 350 ; a= 69; b= 69; c = 0.5