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?

Let's introduce variables for these ages: y, r, v.

y+r+v=42 (Currently, the sum of the ages of Yumi, Rana, and Victoria is 42 years)

r-4+v-4=y (Four years ago, the sum of the ages of Rana and Victoria was equal to the current age of Yumi.)

we add up what we can

r+v=y+8

now, we know that

r+v=y+8 and y+r+v=42

so we can substitute what we know about r+v into the first equation

y + (r+v) = 42

y + (y+8) = 42

2y=42-8

2y=34

y=17

yumi is 17 now!