Hello there!
First, you need to replace that empty space by a letter. As usual, we replace that empty space by x or y. I pick X :)

The missing number is 20
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We can check if we got the answer right. Replace the hole by 20
so it will be
5/12 = 20/48
when divided 5 by 12, it equals to 0.41
when we divided 20 by 48, it equals 0.41
So yes, 20 is the missing number.
I hope this helps you. Please let me know if you have more questions.
Best of luck!