What is 28,550,000,000 rounded to the nearest billion for the   

It might help to think of that given number as 28.55 billion. Dropping all those extra zeros helps simplify things in my opinion. After this point, you round the number 28.55 to the nearest whole number to get the final answer 29 billion. The number 28.55 is closer to 29 than it is to 28. The '5' digit after the 8 tells you to round up rather than down.