Truly it was an insolvable riddle.
"Antony Gray,--Gardener"
Leslie Moore
There is even a certain fascination about the difficult and dangerous, which appeals very strongly to all who know that it is the apparently insolvable difficulty which contains within its bosom the key to the problem which you are seeking to solve.
"In-Darkest-England-and-the-Way-Out"
Booth, William
It was now half past eleven, and after cudgeling his wits for hours, he found the problem of how to extricate himself and Helen from their predicament as insolvable as ever.
"The Gray Phantom's Return"
Herman Landon