I'm having trouble with solve(), take a look at the example. In one example it seems to work fine, but in another, it does not.
56-working fine
55-not working
Any Ideas?
File not found.File not found.

... if you zap units, it works.
That suggests to re-introduce units from bottom to top
up to the point it complains for doctoring.
Cheers . Jean.

Soil Mechanics Quiz - 0503055 - Solution (1) Zombie.sm (55 KiB) downloaded 26 time(s).

Wrote

... if you zap units, it works.
That suggests to re-introduce units from bottom to top
up to the point it complains for doctoring.
Cheers . Jean.

Soil Mechanics Quiz - 0503055 - Solution (1) Zombie.sm (55 KiB) downloaded 26 time(s).

I've tried it without units, still the same result.

By the way, I can't open your file, just crashes every time I try to open it.

Looks like F was not in lbf/ft. Working now... units included.
Soil Mechanics Quiz - 0503055 - Solution.sm (59 KiB) downloaded 32 time(s).

Wrote

I've tried it without units, still the same result.
By the way, I can't open your file, just crashes every time I try to open it.

It's common files don't open.
Yellow, I clicked option ignore Units.
Cheers ... Jean.

Wrote

Looks like F was not in lbf/ft. Working now... units included.

Great ... Be good ... Jean

FindRoot() can work with units and you won't have to enter range.
Just give it a reasonable starting point/guess.
solve() can also be tricked to work with units.
I also showed it in your example.

Regards