Currently, the sum of the ages of yumi, rana and victoria is 42 years. four years ago, the sum of the ages of rana and victoria was equal to the current age of yumi. what is yumi's current age?

Y + R + V = 42
but 4 YEARS AGO : (R-4) +(V-4) = Y

Replace Y in the 1st equation by:  (R-4) +(V-4)

(R-4) +(V-4) + R + V = 42

2R - 2V - 8 = 42;  2R -2V  = 50, or R+V=25
IF R+V =25 AND Y + R +V = 42, then Y + 25 = 42 and Y =17