I still have mine, a yellow one and also a 6" version. I doubt if I remember how to use it. I used it thru college and remember you could tell an engineering student as the case was strapped to our side, just like a cell phone is today. My last semester in college HP introduced the HP35 calculator. IIRC it was about $495. Then the HP45 was introduced and the price of the 35 dropped to $395.How to use this.