JBMO-SL 2018 A5

Let $a, b, c, d$ and $x, y, z, t$ be real numbers such that

$$0 \leq a, b, c, d \leq 1, x, y, z, t \geq 1 \text { and } a+b+c+d+x+y+z+t=8$$

Prove that

$$a^{2}+b^{2}+c^{2}+d^{2}+x^{2}+y^{2}+z^{2}+t^{2} \leq 28$$

When does the equality hold?