(*Batch reactor  A + 2B --> C ; r = kcAcB *)
soln = NDSolve[{cA'[t] == -0.1*cA[t]*cB[t], 
cB'[t] == -0.1*2*cA[t]*cB[t],
cA[0] == 10., cB[0] ==12.},
{cA,cB},{t,0., 5.}];
plot = Plot[{cA[t], cB[t]}/.soln,{t, 0,5}]