Question 1079864
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S = 2R,              (1)
J = S + 10,          (2)
R + S + J = 50.      (3)

The setup is done. You have 3 equations in 3 unknowns.


From (2), J = S + 10 = 2R + 10,     (4)

and you can substitute ((1) and (4) into (3) to get 1 equation for 1 unknown 

R + 2R + (2R+10) = 50.


Now, I am sure, it is an easy exercise for you to complete the solution.
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Good luck!