r = Rosa's age
r + 2 = next child
r + 4 = eldest child
r + (r + 2) + (r + 4) = 36
*Substitute '1' for the Rs
1r + 1r + 1r + 2 + 4 = 36
3r + 6 = 36
*Subtract '6' to both sides
3r + 6 - 6 = 36 - 6
3r = 30
*Divide '3' to both sides
[tex] \frac{3r}{3} = \frac{30}{3} [/tex]
r = 10
Rosa is 10 years old.
