Let j represent Jack's age now. Then:
j = (2/5)(j+20)
Remove the fraction by mult. both sides by 5:
5j = 2(j+20), or 5j = 2j + 40. Solving for j: 3j = 40. Then j = 40/3, or 13 1/3 years.
Must check this result: Is j = (2/5)(j+20) true when j = 40/3 years?
Is 40/3 = (2/5)(40/3+20) true?
LCD is 15. Then (40/3)(15) = (15)(2/5)(40/3 + 60/3), or
200 = 6(100/3). Is this true? Is 200 = 2(100) true? YES.
Jack is 40/3 years old now, or 13 1/3 years.