Question 133910
Let,

x= Bill's age
Now, being Ann as 3 times a sold as Bill at the time when Ann was twice as old as Bill;
3* (2x) = Ann's age
Therefore,
Bill's age + Ann's age = 91
x + 3 (2x) =91
x+ 6x  = 91
x= 91/7
x = 13 , Bill's age now

For Ann's age, 3 (2*13) = 78 yrs old
This just make sense because there was a time when Ann's age was twice as that of Bill, 2*13 = 26. And 3 times this age equals 78, plus 13 (Bill's age now) sum up to 91. Perfect!