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.