Learning math is quite different than learning other subjects, and it is certainly different than learning isolated procedural tasks, such as how to change a flat tire. Math needs to be learned step-by-step. If you do not completely understand a particular topic, and are unable to perform tasks involving that topic with ease, there is no point in moving on to a new topic that depends upon the first one. However, this is exactly what most schools do, although in many cases, they simply do not have a choice.
To test your proficiency at multiplying decimals, do this decimal multiplication quick quiz. For each decimal problem, multiply the two decimals and select the correct answer. Remember it is decimal point multiplication! After multiplying decimals and selecting answers for all 10 decimal problems, check your answers. When you think you've correctly multiplied all 10 decimal multiplication problems, press the Start Set 2 button. Your quiz grade will be shown on the top left of the page. The answers with a check mark are correct. Those with an X are incorrect. The ones with an X will have the correct answers shown.
After you understand how to solve all the decimal multiplication problems, you can go back to the selection page for a new multiplication quiz.