"Hússzor öt az száz."

Translation: Twenty times five is a hundred.

[Kathy979841]

Do I presume egyszer and husszor are to do with matching the vowels again or do you just have to learn these things.

[vvsey]

Yes, it is a vowel harmony thing, like with all suffixes.

[96314081311257]

The ending -szor/szer/ször means "times", as in nyolcszor (eight times), kevésszer (few times), többször (more times) &c.