Correct Answer :    0.8164
                                                                                
                                        Explanation : Given, ratio of rubber engine weight (w) and actual engine weight (W) = 0.8
Now, weight of the rubber engine is given by, w = W * SF1.1
Hence, SF1.1 = w/W = 0.8
1.1*ln (SF) = ln0.8 = -0.223
Hence, ln (SF) = -0.223/1.1 = -0.2028
Hence, SF = e-0.2028 = 0.8164.