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Let x be the Rebecca's age.
Then Jack's age is  3x-5.
From the condition, you have this equation for the sum of ages
    x + (3x-5) = 75.
Simplify and solve
    x + 3x = 75 + 5
     4x    =    80
      x    =    80/4 = 20.
ANSWER.  Rebecca is 20 years old.   Jack is 3*20-5 = 55 years old.
Solved.