n=100000;  # Number of simulated data sets 
c=20;      # Number of components 
h=150;     # Number of Hours
t=15;      # Number need to survive 
lambda=300;
count=0;
for(run in 1:n){
  Time=lambda*rexp(c);
  Surv=Time>=h;
  Nsurv=sum(Surv);
  count=count+as.numeric(Nsurv>=t);
}
Prob=count/n;
sprintf("Approx. Probability: %5.4f",Prob)
p1=exp(-h/lambda);
sprintf("Exact Probaility: %5.4f",1-pbinom(t-1,c,p1))