Suppose that 6 years ago, Alice was four times as old as Betty. In 4 years from now, Alice will only be three times as old as Betty. How old is Betty today.
4b=a then plus 10 to see if the age holds true to 3b=a in ten years. Then since the question is how old is she today? You must either add 6 to young age (of20) or minus 4 from the older (of 30). So Betty would be 26 today.
