A US Billion* has 9 digits following the leading digits (64 being the leading digits in this case.)So 64.1 billion dollars = $64,100,000,000Notice that if you count the number of digits after 64 there are 9Note*: A billion in some other countries is actually a million million (or having 12 digits after the leading digits.) For finance this magnitude of number does not make sense (it's much too large,) and instead the US Billion is used.
6.9 billion as a number would be 6,900,000,000. To write it out without digits it would be six billion and nine hundred million.
To write 87 billion 932, you would start by writing the number 87, followed by the word "billion." Then, you would write the number 932 as digits. So, it would be written as "87 billion 932."
20,000,000
60,000,000
To write 2.1 billion in digits, you would write it as 2,100,000,000. This is because the number 2.1 billion consists of the numeral 2 followed by nine zeros, representing the value of one billion.
29,000,000,000
580,000,000
1,600,000
1,000,000,000
3,810,000,000
1,300,000,000