HP 15c
Great reproduction of the classic! Works great in the iPhone 6. Im an electrical engineer who still depends on my 15c. Main complaint is the very loud clicks for the buttons which is very annoying, especially since there is no way to disable or lower its volume from within the ap. Turning off the ringer on the side of the iPhone to disable sound works but is unacceptable. Hopefully HP will fix this soon - especially if enough of us complain. However, the calculating capabilities work exactly as the original.
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HP 15C Calculator