Question 1113213
probability that 2 work is 7/10*6/9=42/90 or 14/30.  
probability that 1 works and other doesn't is 2*7/10*3/9=42/90 or 14/30
The latter is multiplied by the probability that the next two batteries work, which has a sample space of 8 batteries, 2 of which are defective.  This would be 6/8*5/7=30/56 or 15/28
14/30+(14/30)*(15/28)=14/30+ 210/840=0.717