"An interesting problem."
Translation:Un problema interesante.
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My understanding is that these exceptions come from the word origins themselves. Words like dia and problema are of Greek origin, and this is why they deviate from the otherwise simple Latin rules of thumb. How are you, as a reader, supposed to know? You're expected to memorize, of course!
The patterns are more complex than just -o and -a, and there are a lot of exceptions even when there are patterns. Sometimes a word has been shortened, like la mapa and la foto. Most words that end in -ma are masculine because they were borrowed from Greek. Nouns that end in accented vowels are often masculine: (á, é, í, ó, ú).
Nouns that end in -e are more often masculine, but there are a lot of feminine ones too.
Nouns that end in -d and -z are usually feminine. Nouns that end in other consonants are usually masculine.
I like to look up the history of nouns that are exceptions, because it helps me remember better. Wiktionary is pretty good: https://en.wiktionary.org/wiki/foto#Etymology_12
but sometimes it pays to look further: https://blogs.transparent.com/spanish/word-origins/
Much more here: https://www.spanishdict.com/guide/masculine-and-feminine-nouns
They only seem like many, because we don't notice all the words that match the -o/a rule. The reason why some words don't match that rule is often that the word was adopted from a different language, gender and all. Also, some words are really abbreviation whose full forms have a different suffix.
I'm making up these numbers to make a point. 90% of -a words are feminine. 90% of -ma words are masculine. There are few (or possibly no) patterns that are 100%. You're probably better off memorizing the nouns with an article or with an adjective so you automatically associate the correct gender with each noun.