# Physics Young's Double Slit Problem

In a Young's Double Slit experiment, the separation of four bright fringes is $$2.5\ mm$$, the wavelength of light used is $$6.2 \cdot 10^{-7}\ m$$. If the distance from the slits to the screen is $$80\ cm$$, calculate the separation of two slits.

I applied $$\frac{4\lambda D}{d}=1.5\cdot 10^{-3}\ m$$ but the answer didn't match. It matches when I use $$\frac{3\lambda D}{d}$$ instead of $$4$$.

Shouldn't the separation of four bright fringes be $$\frac{4\lambda D}{d}$$ (after applying $$\sin\theta = \tan\theta$$ approximation)?

$$D$$ :distance from the slits to the screen

$$\lambda$$: wavelength of light used

$$d$$: separation of the slits