Learn Portuguese (Portugal): As viagens para Londres são muito caras.

In Portuguese, words that end in -m in the singular form their plural by dropping the -m and adding -ns. So, a viagem (the trip) becomes as viagens (the trips).

How can I tell that viagens is feminine when it doesn't end in -a?

Most Portuguese words ending in -agem, such as a viagem (the trip), a paragem (the stop), or a mensagem (the message), are feminine. That is why we use the feminine plural article as and the feminine plural adjective caras to match.

If viagens and caras are feminine plural, why doesn't muito change to muitas?

In this sentence, muito means "very" and is describing the adjective caras. When muito means "very", its form never changes. It only changes to match gender and number (like muitas viagens) when it means "many" or "a lot of" and is describing a noun.

Why do we use são (from the verb ser) instead of estão (from estar)?

We use são because the sentence states a general, ongoing fact about trips to London. The verb ser is used for permanent or defining characteristics, whereas estar is used for temporary states.