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Can't understand how to get the answer from (2). Do we consider that dogs and pets are only animals in the shop?

Not so. We are told that \(\frac{1}{3}\) of the pets are dogs and \(\frac{1}{5}\) of the pets are birds --> \(\frac{1}{3}+\frac{1}{5}=\frac{8}{15}<1=total\).

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 shop --> \(\frac{1}{5}\) of the pets are birds --> \(\frac{1}{5}t=30\), where \(t\) is the # of pets at the pet shop --> \(t=150\) --> \(\frac{1}{3}\) of the pets are dogs --> there are \(\frac{1}{3}*150=50\) dogs at the pet shop. Sufficient.

(2) There are 20 more dogs than birds at the pet shop --> \(d=b+20\) --> \(\frac{1}{3}t=\frac{1}{5}t+20\), where \(t\) is the # of pets at the pet shop --> \(t=150\) --> \(\frac{1}{3}\) of the pets are dogs --> there are \(\frac{1}{3}*150=50\) dogs at the pet shop. Sufficient.

Can't understand how to get the answer from (2) Do we consider that dogs and pets are only animals in the shop?

To answer your question, no we do not consider that dogs and birds are the only animals at the shop. In fact, we know they are not because \(\frac{1}{3} + \frac{1}{5} = \frac{8}{15}\). So dogs and birds account for only 8/15th of the total animals.

Now, let's solve this question using ratios.

Dogs : Birds = \(\frac{1}{3} : \frac{1}{5}\). Multiply this ratio by 15 to convert it into integers. Remember, when you multiply the entire ratio by the same number, the ratio remains the same.

Dogs : Birds = 5:3. For every 3 birds, there are 5 dogs. Stmnt 1: If birds are 30, dogs will be 50. Sufficient. Stmnt 2: The difference in number of dogs are birds in ratio terms is 2 (because 5 - 3 = 2), but is actually 20. So 5 will correspond to 50. Sufficient.

Ratios are a great way to quickly and easily solve questions. If this method is not clear, give me a couple of days. I will put up a blog explaining it in detail.
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Does this assume that there are only dogs and birds in the pet shop? If so, shouldn't the problem state it?

If 1/3 of the pets are dogs, and 1/5 of the pets are birds, then only 1/3 + 1/5 = 8/15 of all pets are dogs or birds. So 7/15 of the pets need to be something other than dogs or birds. So no, the question is not assuming there are only dogs and birds in the shop.
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