In a population of $1000$ workers employed at a rugby factory that were exposed to phenyl$-$B$-$naphthylamine$($PBN$),40$ workers developed bladder cancer after $20$ years of occupational exposure and $15$ of them died. In a comparative population of non$-$factory workers of similar age group $,10$ workers were reported to have bladeer cancer. Calculate the risk difference between the two groups