Let x be the amount invested in fund B (the value under the question).


Fund A returned 0.1*15000 dollars, according to the condition.

Fund B returned 0.03*x dollars.


Taken together, the total return is  0.1*15000 + 0.03*x dollars.

From the other side, you are given that it is actually 0.08 of the total investment, which was (15000 + x) dollars.


It gives you an equation

0.1*15000 + 0.03x = 0.08*(15000+x).

Simplify and solve for x:

1500 + 0.03x = 1200 + 0.08x  ====>  1500 - 1200 = 0.08x - 0.03x  ====>  0.05x = 300  ====>  x =  = 6000.


Check.  0.1*15000 + 0.03*6000 = 1680.

        0.08*(15000+6000) = 1680.       ! Correct !


Answer.  Pablo invested  $6000 in fund B.