There are only (10 * 9 * 8 * 7)/(4 * 3 * 2) combinations in10, that is, 2 10 combinations.

There are (10*8*6*4)/(4*3*2) unpaired combinations, that is, 80 combinations.

So there are 130 species with at least 1 pair (including 1 pair and 2 pairs).

Application problem is to describe relevant facts in language or words and reflect some mathematical relationship (such as quantitative relationship, positional relationship, etc.) ) and solved an unknown number of problems. Each application problem includes known conditions and problems.

In the past, China's application problems usually required narrative to meet three requirements: no contradiction, that is, conditions and problems could not be contradictory; Completeness, that is, the condition must be sufficient to ensure that the value of the unknown quantity can be obtained from the condition; Independence, that is, several known conditions cannot be deduced from each other.

Mathematics application problems in primary schools are usually divided into two categories: simple application problems that can be solved in one step only by addition, subtraction, multiplication and division; Complex application problems that need two or more operations to solve.