Problem solving for a variable in Prime

[Snorre]

Hello factor,

OK, then your solve-expression and setting t=40 is totally valid.
Since there doesn't exist a closed form solution to that integral (I think that's the reason we have the approximate erf and CDF functions for), you should take a closer look at NORMALD_CDF (which gives the integration over the left side). But to be sure consult the on-device-help.
Your desired solution may look like: solve(1-normald_cdf(u,s,t)=0.1,s).