Start both hourglasses then when the 11 minute hourglass has finished immediately flip it again. When the 13 minute hourglass runs out the 11 minute hourglass will have 9 minutes left, so flip it and it will last another 2 minutes, 13 minutes + 2 minutes = 15 minutes.
"Great Job You Got It"
This riddle is named after Julius Caesar, who is thought to be first to use this cipher.
Simply count the letters and arrange them in groups of the same number, here 4 will be easiest. It will be make 4 rows of 4. Then you put the rows one above the other and read up down, like so:
G T Y O
R J O T
E O U I
A B G T
Until you find the message.