To solve this question, we begin with 12.4. The only operation you need to use for this problem is addition. We will slowly add 0.2 to 12.4 until we reach 13.8. As you add, be sure to align the decimal points! Also, remember to keep track of your numbers, as you will need them to answer the question.
12.4+0.2=12.6. 12.6+0.2=12.8. 12.8+0.2=13.0. 13.0+0.2=13.2. 13.2+0.2=13.4. 13.4+0.2=13.6. 13.6+0.2=13.8.
So, the decimals are (including 12.4 and 13.8): 12.4, 12.6, 12.8, 13.0, 13.2, 13.4, 13.6, and 13.8.
Hope this answered your question!