Answer:
Mi pobre hermano está enfermo y no puede ir al concierto este fin de semana. Yo voy a ir <u>por</u> él para no perder el dinero.
Explanation:
The translation for these sentences is:
My poor brother is sick and cannot go to the concert this weekend. I'm going to go <u>instead of him</u> so we don't lose money.
Por is a preposition. Prepositions give us information abou other words in the sentence (like time and place). Much like <em>para</em>, both prepositions share similar meanings and diverse usages. <em>Por</em> can generally be translated to <em>for</em> (Lo hice <u>por</u> él / I did it <u>for</u> him) but that is not always the case since para can be used like that too.
In the sentence provided by the exercise, <em>por</em> means<em> instead of. </em>If I had used <em>para</em> in this particular sentence (Yo voy a ir para él para no perder dinero) it would have meant "for him" and not "instead of him".