To solve this equation, we first need to simplify the expression inside the parentheses by adding the terms:

6.31 + 2.59 = 8.9

Next, we multiply this sum by 2.5:

8.9 * 2.5 = 22.25

Now let's simplify the equation given:

2.8 + у + 3.7 = 125

Combine the numbers:

6.5 + у = 125

Subtract 6.5 from both sides:

у = 125 - 6.5
у = 118.5

Therefore, the solution to the equation is у = 118.5.