I think the practice question could have been more specific, like:

What is the value of 54 ones x 10 expressed in tens?

As for finding the answer:

You start off with 54, which means you have 4 ones and 5 tens:

         tens ones
            5    4

Now, you multiple by ten which means everything will shift one position to the left and you pad the empty ones place with 0:

hundreds tens ones
       5    4    0

So, the result is:

• 5 hundreds and 4 tens, or
• 54 tens (if you want to express the result in tens (which seems to be what they want), or
• 540 ones

I hope my explanation was helpful and did not confuse you more :)