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My textbook [Japanese from zero 1, George Trombley] gives 6 and 8 as roku mai
and hachi mai.

But then gives 16 and 18 as じゅうろっまい juu rommai and じゅうはっまい juu hammai.

Is this correct ?

I am only learning but it sounds wrong to me. I would imagine it would be juu
roku mai and juu hachi mai.

If so then does 26/28, 36/38, etc. follow the same pattern ?
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2020-12-23T19:42:41.020
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2020-12-23T22:38:25.020
2020-12-23T21:14:35.140
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pronunciation counters
How would you say 16 and 18 with the counter mai?
As you noted, 6枚 should be read as ろくまい, and 8枚 should be read as はちまい.

Similarly, 16枚 should be read as じゅうろくまい, and 18枚 should be read as じゅうはちまい.

I have no idea why your textbook would render these as じゅうろっまい and じゅうはっまい.
Neither pronunciation is correct -- these aren't even dialectal variants, so
far as I'm aware; they're just wrong.

PS: The larger numbers you asked about, like 26枚・28枚 etc, would still use the
standard ～ろくまい・～はちまい pronunciations.
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2020-12-23T20:42:07.053
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answer