Use + or - Mario earned three times as much money last year as his wife Elena. The total amount that the two earned was $11,000. How much money did Elena earn?

Let's assume that Mario's income is "m" and Elena's income is "e"

m = 3e

m + e = 11,000

3e + e = 11,000 ---> 4e = 11,000 ----> e = 11000/4 ---> e = **$2,750**