According to the meaning of the question, there are 18 people who can sing and 20 people who can dance.

Then calculate the number of people who can sing and dance by addition.

Column, 18+20=38,

There are 28 students in this class, and the number of people who can sing and dance can be calculated by subtraction.

Column, 38-28= 10,

So there are 28 people in Class Three One, among whom 18 can sing and 20 can dance, so 10 can sing and dance.

Extended data:

This kind of problem belongs to the set problem in mathematics. The steps to solve this kind of problem are as follows:

1. Calculate the total number of two types of values through addition;

2. Calculate the difference between the two sums and the actual sum by subtraction;

This difference is the answer to this group of questions.

Addition and subtraction algorithm

1, integer:

(1) Same number alignment

(2) Counting from the unit

(3) When it is added to dozens, it will be pushed to a higher position; If the subtraction is not enough, subtract 1 from the high position. When this number is added to 10, it will decrease.

2. Decimal number:

(1) decimal point alignment (that is, same digit alignment);

(2) calculation according to the law of integer addition and subtraction;

(3) aim the decimal point at the horizontal line in the number and point the decimal point;

Step 3 score

(1) Add and subtract fractions with the same denominator, and only add and subtract numerators;

(2) For the addition and subtraction of fractions with different denominators, the division operation is carried out first, and then the calculation is carried out according to the addition and subtraction algorithm of fractions with the same denominator;

(3) The quotation whose result is not the simplest score is divided into the simplest score.