HP 10bii+ Margin/Markup bug

[Rick314]

The HP 10bii+ has MU CST PRC MAR (markup, cost, price, margin) keys. Only certain entry and solve-for sequences are allowed when mathematically the solutions are available. For example (ignoring percent conversions) MAR = MU/(1 + MU). But entering MU and solving for MAR gives an error. Similarly, MU = MAR/(1 - MAR). But entering MAR and solving for MU gives an error. As another example, from page 48 of the user manual, enter 9.6 CST 15 MU and solve for PRC = 11.04 and MAR = 13.04 (OK so far). But start over (C ALL) and try to solve for MAR before solving for PRC. You get an error, even though MAR is known from the entry of MU alone. My guess is that this is because of the way the firmware is using the HP SOLVE app that is inside many of their calculators. They are not keeping track of all possibilities of what can be determined from the values provided. I don't have an HP 10bii (without the +) but would guess the problem exists there too. Or am I misunderstanding something?