0.48/2.4 = erfc(15.05/(2sqrt(D49/24)))
It sometimes seems happier solving for x than for another name
Ah, thank you! You've rescued me from my own stupidity!
D is a protected symbol, used for differentiation.
Not for WolframAlpha, at least sometimes
WA successfully solving for D
However, without the space between D and the trailing 49/24 it consistently times out, says it can't do it and THEN gives a vast long list of complex numbers as the solution, while with the space between D and the trailing 49/24 it happily quickly finds what is likely the desired real valued solution...
at least this time.
D 47 means D * x where D49 would be a variable name in Mathematica. If you use D49 free form input, it seems to interpret it as D^49
With the desktop publishing of the result from
solve 0.48/2.4 = erfc(15.05/(2sqrt(D49/24))) for D
I didn't even notice the slightly elevated 49 on the WolframAlpha page was denoting that this was the 49th power of D. Trying lots of other problems with x2 or even D3 makes the exponent much more obvious.