Learn Spanish (Spain): ¿Queréis comprar cuarenta manzanas?

No. The numbers 30 through 90 don't change to match the gender of the noun they describe, and they don't take an extra 's' for plural. You just use cuarenta whether you are counting masculine objects or feminine ones like manzanas.

How is cuarenta (forty) related to the word for four (cuatro)?

Just like "four" and "forty" in English, they look similar but have a slight spelling difference. The "u" from cuatro is dropped to make cuarenta. Notice that many of the "tens" in Spanish end in -enta, such as cincuenta (50) and ochenta (80).

Who does queréis refer to in this sentence?

Queréis means "you all want". It is the vosotros form of the verb querer (to want). This is the informal plural way to say "you" that is used primarily in Spain.

Why doesn't the verb comprar change its ending to match "you all"?

In Spanish, when two verbs are used together for a single action (like "want to buy"), only the first verb changes its ending to match the person doing the action. The first verb becomes queréis (you all want), while the second verb stays in its original dictionary form, comprar (to buy).