Answer:
S/ 40.
Explanation:
Although the question is in Spanish, I am going to explain you the answer in English, since this question is in the mathematics section and not Spanish.
The word statement must be translated into mathematical (algebraic) language.
1) First, name the variable or variables:
x: the amount of money you have today.
2) Statement: "Anteayer tuve el triple de lo que tengo hoy" ⇒ 3x
3) Statement: "Lo que tenía ayer, fue S / 50 menos que anteayer":
⇒ 3x - 50
4) Statement: "Lo que tengo hoy es el doble de lo que tenía ayer":
⇒ x = 2 (3x - 50)
5) Now you can solve the equation:
a) Distributive property: x = 6x - 100
b) Addition property of equality: x + 100 = 6x
c) Subtraction propery of equality: 100 = 6x - x
d) Simplification: 100 = 5x
e) Division property of equality: 100/5 = x
f) Simplification: 20 = x
Which means that you have today S/ 20.
6) The final question is how many Soles do I need to buy a trouser that costs S/ 60.
Since, you have S/ 20, you need the difference to S/ 60:
S/ 60 - S/ 20 = S/ 40
Therefore, the answer is S/ 40