Question: Multiples of 10 in Tagalog?
Names for multiples of 10 in Tagalog are formed from the corresponding unit and a suffix which I guess denotes a group of 10 items. That's obvious in dalawampu (20) from dalawa (2), tatlumpu (30) from tatlo (3), apatnapu (40) from apat (4), limampu (50) from lima (5), animnapu (60) from anim (6), pitumpu (70) from pito (7), walumpu (80) from walo (8), and siyamnapu (90) from siyam (9). I believe even sampu (10) may follow that pattern, from isa (1) with the initial i getting elided at some point.
I can't help wonder why the group of 10 morpheme has two forms, -(a)mpu and -napu. It's pretty clear to me that -(a)mpu is used with a units name ending in a vowel (with merged vowels) and -napu with one ending in a consonant, but if they're cognates or alternate forms, why the mp/p alternation, and is one derived from the other, do they have a common ancestor with one undergoing metathesis, or is the similarity a coincidence? Perhaps relatedly, the (vowel)-mpu compounds have final-syllable stress only, and the -napu compounds (except for siyamnapu, perhaps because "siyam" is usually pronounced as a single, merged syllable) have both initial-syllable and final-syllable stress. Would that stress pattern help explain the change?
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