GMAT question 47

Bobby and his younger brother Johnny have the same birthday. Johnny's age now is the same as Bobby's age was when Johnny was half as old as Bobby is now. What is Bobby's age now?  (1) Bobby is currently four times as old as he was when Johnny was born.  (2) Bobby was six years old when Johnny was born.

Solution:
One way to solve this problem is to realize that, as two people age, the ratio of their ages changes but the difference in their ages remains constant. In particular, the difference in the boys ages "now'" must be the same as the difference in their ages "then". This leads to the equation: y - x = x - (1/2)y, which reduces to x = (3/4)y; Johnny is currently three-fourths as old as Bobby.   Without another equation, however, we can't solve for the values of either x or y. (Alternatively, we could compute the elapsed time between "then" and "now" for each boy and set the two equal; this leads to the same equation as above.)   (1) INSUFFICIENT: Bobby's age at the time of Johnny's birth is the same as the difference between their ages, y - x. So statement (1) tells us that y = 4(y - x), which reduces to x = (3/4)y. This adds no more information to what we already knew! Statement (1) is insufficient.   (2) SUFFICIENT: This tells us that Bobby is 6 years older than Johnny; i.e., y = x + 6. This gives us a second equations in the two unknowns so, except in some rare cases, we should be able to solve for both x and y -- statement (2) is sufficient. Just to verify, substitute x = (3/4)y into the second equation to obtain y = (3/4)y + 6 , which implies y = 24. Bobby is currently 24 and Johnny is currently 18.
Answer is Bc