Hey there!
It kind of depends if every single person at the market place ends up looking at Aladdin, but here's how you would solve this problem if every single person at the marketplace was looking at Aladdin. . .
Since there are 2 eyes on one head, divide the numbers 20 and 2 . . .
20 divided by 2 = 10 <-------
The answer to your question is 10.
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There were 10 people at the marketplace that day.
Hope this helps you.
Have a great day!