Left shift 3 times the number 2 and need a way to get 15

I have met with a mathematical question which needs to fill blanks as below.

Take the number two. Then ___________ it 3 times and do ___________ to make it 15.

So what I suggested is left shift 2 by 3 times and get 16. But then I could not found a mathematical way to make it 15

Please explain your ideas.