Pizarras
Unas manos is the plural form of "una mano".
Unas clases
It's "unas fotografías."
It's, "las pizarras.
It's never the same. Could be unas, unos, los, las, etc.
masculine: un, unos feminine: una, unas
Unas gomas.
The singular form of "unas libretas" is "una libreta."
The singular form of "unas capitales" is "una capital."
Unos - masculine objects Unas - feminine objects
masculine = un (a/an, singular), unos (some, plural) feminine = una (a/an, singular), unas (some, plural)