Final answer:
To find the missing number in a proportion, cross multiply and solve for the variable.
Step-by-step explanation:
To find the missing number, you can set up a proportion between the given percentage and the whole number. In this case, the equation would be: x/400 = 280/100. To solve for x, cross multiply and divide to get x = (280 * 400)/100 = 1120. Therefore, the missing number is 1120.
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