According to the three pictures given, we need to complete the following addition and subtraction problems.
Figure 1: Four children are skipping rope.
Figure 2: There are two children left.
Figure 3: There are four children.
First, let's take a look at the figure 1. There are four children skipping rope. This means that there are four children at the beginning.
Next, let's look at Figure 2, in which there are two children walking. This means subtracting two from the original four children. So, we need to do subtraction: 4-2=2.
Finally, let's look at Figure 3. There are four children in the photo. This means that after the subtraction in Figure 2, the number of children left is 4. So we need to add: 2+2=4.
To sum up, according to the three pictures given, the topic of addition and subtraction we need to complete is: 4-2+2=4. The answer is 4.
The purpose of this topic is to let the first-year students understand the concept of addition and subtraction by observing numbers and make simple calculations. Through such exercises, students can cultivate their mathematical thinking ability and calculation ability. At the same time, it is also an effective way to cultivate students' observation and logical thinking ability.