Jack is 13 years older than Bao. In 8 years, he will be twice as old as Bao. How old is Jack now?
So we can express jacks curent age as x, and bao's current age as x-13. It says that 'In 8 years, he will be twice as old as Bao" and we can express that as x+8=2(x-13+8) which we can simplify to x+8=2x-10 from here x=18, so jack is curently 18.