When you round something, is usually the number under 4 (rounded down) or 5 and up (rounded up). This can be done to tens, hundreds, thousands, etc.
Here, look at the hundreths place. The number is under 4, so we round down, right? Not quite. The full number is 368. The number is closer to 400 than 300. So, we round up to 400.
The number you would write is 51,400.
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