days=2000;
walk.to.bus = rnorm(days,mean=4,sd=1); summary(walk.to.bus);
wait.for.bus = runif(days,4,16); summary(wait.for.bus);
bus.ride = rnorm(days,mean=20,sd=2); summary(bus.ride);
walk.up.hill = rgamma(days,scale=2,shape=3/2); summary(walk.up.hill);
total.time = walk.to.bus + wait.for.bus + bus.ride + walk.up.hill;
summary(total.time);
c(mean(total.time),sd(total.time),var(total.time))
hist(total.time)
boxplot(total.time)

# sum of 12 uniforms

r = function(dummy) {sum(runif(12))-6} ; 
sum.12.uniforms = sapply(1:50000,r); 
hist(sum.12.uniforms,breaks=50)