There were many correct solutions to this problem: Alex from Stoke on Trent Sixth Form; Manuele and Eduardo, both from the British School, Manila; Jeff from Sierra Vista; Dapeng from Claremont Fancourt School, Diarmuid from Trinity School, Nottingham; Christina from Smithdon High School; Thomas from A Y Jackson School; Tathagata from South Point High School; Jack from Guthlaxton College; Innyeong Chang from Nanjing International School; Chen Jinsheng from ACS Independent School and John from Takapuna Grammar School.

This is Alex's solution:

Multiply each side of the inequality by $\sqrt x$. There is no problem about reversing the inequality sign, because $\sqrt x$ is given as positive. This gives: $x + 1 < 4\sqrt x$.

Subtract $4\sqrt x$ from both sides so the right-hand side is zero to give the following inequality involving a quadratic in $\sqrt x$:

$$x - 4\sqrt x + 1 < 0.$$

Use the quadratic formula to factorise the left-hand side:

$$(\sqrt x - 2 -\sqrt 3)(\sqrt x - 2 + \sqrt 3) < 0.$$

Considering the value of the left-hand side for the positive values of $\sqrt x$, the left-hand side is less than zero as required only for the range

$$2 - \sqrt 3 < \sqrt x < 2 + \sqrt 3.$$

Here $x$ and $\sqrt x$ are both positive, and each value of $\sqrt x$ corresponds to a sole $x$ value and vice-versa, so everything in the inequality can be squared without losing solutions. The solution of the inequality is:

$$7-4\sqrt 3 < x < 7+4\sqrt 3.$$

Note that $7-4\sqrt 3\approx 0.0718 > 0$.

Christina's method was slightly more direct:

Square both sides. Multiply throughout by x. Rearrange to form the quadratic inequality:

$$x^2-14x+1< 0.$$

Use the quadratic formula to solve this inequality. From the graph of $y =x^2 -14x + 1$ we see that the solution is $7-4\sqrt 3 < x < 7+4\sqrt 3$ or approximately $0.072< x< 13.928.$