Call Yumi's age is a

Rana's age is b

Victoria's age is c.

We have: a + b + c = 42 

=> ( a - 4 ) + ( b - 4 ) + c = 36

But a - 4 + b - 4 = c 

=> a + b = c + 8 (1)

We also have: a + b + c = 42

=> a + b = 42 - c (2)

From (1) and (2) we have:

42 - c = c + 8

=> 42 - 8 = 2c

=> 36 = 2c

=> c = 18

So current Yumi's age is 18.