to the firm one:P1=1000-5*100=500;when the demand increase one unit, the price becomes 1000-5*101=495, so the demand-price elasticity should be -[(495-500)/500]/[1/100]=;and the demand-price elasticity for the other firm has the same solution

to the firm two:P2=1600-4*250=600;when the demand increase one unit, the price becomes 1600-4*251=596, so the demand-price elasticity should be -[(596-600)/1]*[600/250]=;