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Answer 1 · 2017-10-18T18:09:01

See a solution process below{

Let's call the age of the younger sister: #a#

Then, the age of the older sister is: #2a#

If the sum of their ages is #12# we can write this equation and solve for #a#:

#a + 2a = 12#

#1a + 2a = 12#

#(1 + 2)a = 12#

#3a = 12#

#(3a)/color(red)(3) = 12/color(red)(3)#

#(color(red)(cancel(color(black)(3)))a)/cancel(color(red)(3)) = 4#

#a = 4#

The younger sister is: 4 years old.

The older sister is: #2 xx 4 = #8 years old.