Without hesitating I blurted out "85". The young man next to me asked "how did you know that?" lol
That’s easy. I multiply everything by 10 and then go from there. The 10 makes it easier than the 5.
17x10=170 divide by 2 since 5 is half of 10.
When I was explaining this to granddaughter she got a giant blank look on her face. After I kept throwing numbers at her and she did it a few times you could see the lightbulb go off.