1) At a certain pet shop, 1/3 of the pets are dogs and 1/5 of the pets are birds. How many of the pets are dogs?
1. There are 30 birds at the pet shop
2. There are 20 more dogs than birds at the pet shop

The answer is D, but I don't know why.

2)On a certain sight-seeing tour, the ratio of the number of women to the number of children was 5 to 2. What was the number of men on the sight-seeing tour?

1. On the sight seeing tour, the ratio of the number of children to men was 5 to 11.
2. The number of women on the sight-seeing tour was less than 30.

Statement number one combined with the stimulus is clearly sufficient. If we know that there are 30 birds at that this represents 1/5 of the total pets we know that there are 150 total pets. Multiply by 1/3 to see that there are 30 dogs.

Statement 2 is a little trickier. If there are 20 more dogs than birds you can set up an equation. Let T stand for total pets. You can say that 1/3(T) (this is the number of dogs) minus 1/5(T) (the number of birds) = 20. So that is 1/3(T) - 1/5(T) = 20. Get a common denominator and it is 5/15 (T) - 3 (15) T = 30. So 2/15 (T) = 20. Multiply by 15 and divide by 2 and we see that T = 150. Multiply by 1/3 and you get 50 dogs.