Deductive reasoning is what is proven and what you already know for sure. Example: there are 2 ants. There is 1 ant next to the 2. There are 3 ants total. (Rules of addition)
So you don't have to solve any problems to use deductive reasoning, unlike inductive reasoning, which is not proven
Example of inductive reasoning: there are lots of girls in my class. So there are probably lots of girls in the other classes too. You don't know that for sure.