Let S (α)={(x, y): y^2 ≤ x, 0 ≤ x ≤ α} and A (α) is area of the region S(α). If for a λ, 0<λ<4, A (λ): A (4)=2: 5, then λ equals:

Let $\displaystyle \mathrm{S}(\alpha)=\left\{(x, y): y^2 \leq x, 0 \leq x \leq \alpha\right\}$ and $\displaystyle \mathrm{A}(\alpha)$ is area of the region $\displaystyle S(\alpha)$. If for a $\displaystyle \lambda, 0<\lambda<4$, $\displaystyle \mathrm{A}(\lambda): \mathrm{A}(4)=2: 5$, then $\displaystyle \lambda$ equals :