Given that this is based on simple interest, in the first case, investing at 4% for 3 years gives a total interest of 4%*3 = 12%. In the second case, investing at 6% for 4 years gives a total interest of 6%*4 years = 24%. (We can do this since the same principal is given to both.) This is exactly double that of the first case of 12%, so the second choice is the right answer.
