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Part of the answer to this depends on whether you are learning the American versus British English variant.

In British English, one would say

  That cake had one hundred calories in it

That cake had one hundred calories in it

In American English, one would more typically say

  That cake had a hundred calories in it

That cake had a hundred calories in it

Additionally there are other differences between how numbers are stated in British versus American English that often cause confusion for new learners:

 This bicycle cost two thousand, one hundred thirty seven dollars

This bicycle cost two thousand, one hundred thirty seven dollars

Would be a valid American englishEnglish number -, i.e. $2137, whereas in British English one would preferentially use the form

 This bicycle cost two thousand, one hundred **and** thirty seven pounds.

This bicycle cost two thousand, one hundred and thirty seven pounds.

Meaning the same number - £2137.

In both American and British English, you need to qualify the number with "a" or "one" when stating an exactitude, for example

 This book has hundred pages

This book has hundred pages

Is wrong in both American and British English. However, you can use the plural form of the quantity without "a" or "one" for indefinite quantities to express an order of magnitude:

 This book has hundreds of pages

This book has hundreds of pages

Which would be used either idiomatically to mean

 This book has lots of pages

This book has lots of pages

Or precisely to mean that the book has somewhere between 100-999 pages.

Part of the answer to this depends on whether you are learning the American versus British English variant.

In British English, one would say

  That cake had one hundred calories in it

In American English, one would more typically say

  That cake had a hundred calories in it

Additionally there are other differences between how numbers are stated in British versus American English that often cause confusion for new learners:

 This bicycle cost two thousand, one hundred thirty seven dollars

Would be a valid American english number - i.e. $2137, whereas in British English one would preferentially use the form

 This bicycle cost two thousand, one hundred **and** thirty seven pounds.

Meaning the same number - £2137.

In both American and British English, you need to qualify the number with "a" or "one" when stating an exactitude, for example

 This book has hundred pages

Is wrong in both American and British English. However, you can use the plural form of the quantity without "a" or "one" for indefinite quantities to express an order of magnitude:

 This book has hundreds of pages

Which would be used either idiomatically to mean

 This book has lots of pages

Or precisely to mean that the book has somewhere between 100-999 pages.

Part of the answer to this depends on whether you are learning the American versus British English variant.

In British English, one would say

That cake had one hundred calories in it

In American English, one would more typically say

That cake had a hundred calories in it

Additionally there are other differences between how numbers are stated in British versus American English that often cause confusion for new learners:

This bicycle cost two thousand, one hundred thirty seven dollars

Would be a valid American English number, i.e. $2137, whereas in British English one would preferentially use the form

This bicycle cost two thousand, one hundred and thirty seven pounds.

Meaning the same number - £2137.

In both American and British English, you need to qualify the number with "a" or "one" when stating an exactitude, for example

This book has hundred pages

Is wrong in both American and British English. However, you can use the plural form of the quantity without "a" or "one" for indefinite quantities to express an order of magnitude:

This book has hundreds of pages

Which would be used either idiomatically to mean

This book has lots of pages

Or precisely to mean that the book has somewhere between 100-999 pages.

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Part of the answer to this depends on whether you are learning the American versus British English variant.

In British English, one would say

  That cake had one hundred calories in it

In American English, one would more typically say

  That cake had a hundred calories in it

Additionally there are other differences between how numbers are stated in British versus American English that often cause confusion for new learners:

 This bicycle cost two thousand, one hundred thirty seven dollars

Would be a valid American english number - i.e. $2137, whereas in British English one would preferentially use the form

 This bicycle cost two thousand, one hundred **and** thirty seven pounds.

Meaning the same number - £2137.

In both American and British English, you need to qualify the number with "a" or "one" when stating an exactitude, for example

 This book has hundred pages

Is wrong in both American and British English. However, you can use the plural form of the quantity without "a" or "one" for indefinite quantities to express an order of magnitude:

 This book has hundreds of pages

Which would be used either idiomatically to mean

 This book has lots of pages

Or precisely to mean that the book has somewhere between 100-999 pages.