A box of 6 trophies costs 5$. How much would it cost to buy 48 trophies ?

Ah, this is simple multiplecation, on both sides. 
You have a box of 6 trophies, and it costs 5 bucks.
Well, you should know that 6*8=48
So, if 6*8=48 trophies, then multiply 5 bucks *8, which gives you 40

$40 for 48 trophies.

~Hope this helps!