HP-35 Calculator 1972 Hewlett-Packard Corporation, United States Despite market studies that predicted little demand for such a device, HP co-founder Bill Hewlett challenged his engineers to create a calculator as powerful as a desktop model, but that would be small enough to fit in his shirt pocket. Even with its $395 price tag, the HP-35 was an overwhelming success. As prices dropped, the engineer's beloved slide rule quickly became obsolete. In an unusual move, HP-35 used "Reverse Polish" notation instead of mathematical expressions grouped with parentheses. Thus "3 x (4 +5) was entered as "4 5 + 3 x".