First, let's make this easier. Think of 3/5 multiplied by 10/21 as different multiplication problems. First, you should multiply the numerators then the denominators. But before we actually multiply, we can cross simplify.
3 10
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5 21 3 divided by 3= 1. 21 divided by 3 is 7. Then you can cross simplify again. 5 divided by 5 is 1. 10 divided by 5 is 2. So your multiplication problem has now been put into simpler terms and smaller numbers. This makes it easier to solve so you don't have to multiply such big numbers together.
Your new much simpler multiplication problem is 1/1 multiplied by 2/7. Your answer is 2/7 and it has been put into simplest form.
(Sorry the explanation is so long. Hope this helps!)
