Excel forgotten how to add

You have a floating point error

floating point error
https://support.microsoft.com/en-gb/...sults-in-excel

https://www.microsoft.com/en-us/micr...wrong-answers/

https://www.excel-easy.com/examples/...nt-errors.html

so you need to use a round()
=ROUND(B2+0.1,1)

Whilst your explanation makes sense I don't think it solves the problem.

as you will see from the links I posted or if you search for Excel Floating Point Error - you will see this is a very very common issue
Formatting the cell , will make no difference to the result , as that is the number that is displayed NOT the number that is actually contained in the cell - So even though thats what MS may have said to you - as you have found out , its not actually a solution for some applications of excel.

Again those articles I posted actually show that error and impact when used in any logical test - for TRUE & FALSE, you get the WRONG result, since displaying to 2 decimal places does NOT actually resolve whats contained within the actual cell.

I guess you have been lucky not to have come across this in 20 years of using excel or perhaps it did happen, but in the application you have been using was not visible or an issue

I don't think it reasonable for Microsoft to use the excuse on number standards.

agree, BUT it a known error, and you will need to work around it, as excel will not be changed anytime soon.

I have been using excel since version 4 about 1992 and before that lotus 123 from about 1987 - the first time i came across the error was in 2006 when I was doing a highly complex financial currency exchange conversion across about 30 countries and different currencies for one of the largest global IT company.

I agree with etaf - this is a long-standing known problem, with quite a few ways to work around it. Without changing the whole underlying mechanics that excel is based on - which is about as likely as me falling pregnant (Im a guy btw), there is no way this will be rectified. It is not a "excel cannot add" problem, it is a fundamental underflow/overflow situation with binary math

Microsoft Excel was designed around the IEEE 754 specification to determine how it stores and calculates floating-point numbers. IEEE is the Institute of Electrical and Electronics Engineers, an international body that, among other things, determines standards for computer software and hardware. The 754 specification is a very widely adopted specification that describes how floating-point numbers should be stored in a binary computer. It is popular because it allows floating-point numbers to be stored in a reasonable amount of space and calculations to occur relatively quickly. The 754 standard is used in the floating-point units and numeric data processors of nearly all of today's PC-based microprocessors that implement floating-point math, including the Intel, Motorola, Sun, and MIPS processors.

So, it is a problem with the system excel is based on, not a problem with excel.