We have a data set here to correctly or properly arrange using bubble sort, and also listing the intermediate steps. The set of number is 15, 20, 10, 18. To use a bubble sort, you have to arrange your number from the least of the highest, but it has to follow a process in such a way that only the numbers that are beside themselves can be switched, but no number can jump the other.

So, in the process of switching through numbers, you are aiming at have the numbers arranged in ascending order. Now, having these explanations in mind and having understood them, let us get back to our set of number, which is 15, 20, 10, 18. Now the smallest number there is 10.

So let us switch it by taking it leftward. We have 15, 10, 20, 18. Mind you, you can only switch two numbers at a time. Next, we will be switching 15 and 10, then we have 10, 15, 20, 18. And finally, we will have 10, 15, 18, 20. Thus, the correct intermediate steps would be 15, 10, 20, 18... 10, 15, 20, 18... 10, 15, 18, 20.