I know this is an old forum but this is for anyone who still needs it. The answer on solution bank isn't complete - they forgot to multiply 13/18 by 1.5.
0.722 x 1.5 = 1.08
So just follow the one on solution bank and finish the last step. The key here is that you have got to assume that the width is 0 - 1.5. To be honest I'm still stuck on why this is, since the data isn't "to the nearest week", it is simply after each week.