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!
