Question 1072161
Ellie is twice as old as her sister kate. In seven years the sum of their ages will be 20. How old are they now?
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Let E = Ellie's age
and K = Kate's age

E = 2K  (1)
(E+7)+ (K+7) = 20   (2)

Substitute 2K for E from (1), into (2):
(2K+7) + (K+7) = 20
     3K + 14 = 20
       3K = 6
         K = 2   —>  E = 2K = 2*2 = 4

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Ans:  Kate is 2 years old, Ellie is 4 years old
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Check:   in 7 years,  Kate will be 9 and Ellie will be 11, and 9+11 = 20, so that check ok.