13 books were given to Dongdong, and there are still 10 books left.

There are as many at this time, so Dongdong gets three copies before it reaches 10.

That is 10-3=7 copies.

Dongdong originally had seven books.

Extended data

Significance of one-dimensional linear equation

One-dimensional linear equation can solve most engineering problems, travel problems, distribution problems, profit and loss problem, integral table problems, telephone billing problems and digital problems. If only arithmetic is used, some problems may be extremely complicated and difficult to understand. The establishment of linear equation model will be able to find the equivalence relationship from practical problems and abstract it into a mathematical problem that can be solved by linear equations.

For example, it may not be possible to start with algebraic expressions for the Diophantine problem, but finding "age" as an equivalent relationship through a linear equation will simplify the problem. One-dimensional linear equations can also play a role in proving mathematical theorems, such as proving that "the period of 0.9 is equal to 1" within the scope of elementary mathematics.

By verifying the rationality of the solution of linear equation, the purpose of explaining and solving life problems is achieved, and some problems in production and life are solved to some extent.

Baidu encyclopedia-unary linear equation