The current of 5 A flows in a square loop of sides 1 m is placed in air. The magnetic field at the centre of the loop is $$X\sqrt{2} \times 10^{-7}$$ T. The value of X is _________.

Magnetic field at center of a square loop of side $$a$$ with current $$I$$:

Each side contributes $$B_1 = \frac{\mu_0 I}{4\pi d}(\sin\alpha_1 + \sin\alpha_2)$$ where $$d = a/2$$ and $$\alpha_1 = \alpha_2 = 45°$$.

$$B_1 = \frac{\mu_0 I}{4\pi (a/2)} \times 2\sin 45° = \frac{\mu_0 I}{2\pi a} \times \sqrt{2} = \frac{\sqrt{2}\mu_0 I}{2\pi a}$$

Total for 4 sides: $$B = 4B_1 = \frac{4\sqrt{2}\mu_0 I}{2\pi a} = \frac{2\sqrt{2}\mu_0 I}{\pi a}$$

With $$I = 5$$A, $$a = 1$$m:

$$B = \frac{2\sqrt{2} \times 4\pi \times 10^{-7} \times 5}{\pi \times 1} = 2\sqrt{2} \times 4 \times 5 \times 10^{-7} = 40\sqrt{2} \times 10^{-7}$$

So $$X = 40$$.

Therefore, the answer is $$\boxed{40}$$.