The likelihood of selecting a lens that is defective is determined by two factors:
1) The likelihood of selecting one company over another
2) The percent chance of getting a defective lens from that company.
Probability of selecting a Greens Lens: 60/100
Probability of selecting a Parsons Lens: 15/100
Probability of selecting a Ratten Lens: 25/100
So, from that list, we then multiply against the defect rate to determine the likelihood of selecting a company AND a defective lens:
Probability of selecting a defective Greens Lens=(3/5)*(1/25)=3/125
Probability of selecting a defective Parsons lens=(3/20)*(1/10)=3/200
Probability of selecting a defective Ratten Lens=(1/4)*(3/50)= 3/200
I cannot tell from the answer you posted whether or not you needed defective or non-defective rates, so here are the nondefective probabilities:
Probability of selecting a NON-defective Greens Lens=(3/5)*(11/20)=33/100
Probability of selecting a NON-defective Parsons lens=(3/20)*(1/20)=3/400
Probability of selecting a NON-defective Ratten Lens=(1/4)*(19/100)=19/400