Most word problems don't tell you whether to add, subtract, multiply or divide. You have to think about the problem and decide what to do. It may help to highlight key words, and draw a diagram or sketch. Look at this example:

There are six cans of drink in a multipack.  How many cans are there in three packs?

Now look at the diagram - three lots of six. There are two ways to work out how many cans:

3 × 6 = 18 cans
6 + 6 + 6 = 18 cans