Official solution
\[\bar x = \frac{30+70+25x}{4+x} = 25 \quad(\text{based on recovered class-mark and frequency data})\]
\[\sigma^2 = \frac{\sum f_ix_i^2}{\sum f_i}-\bar x^2 \implies 50 = \frac{450+625x+2450}{4+x}-625\]
\[675(4+x) = 2900+625x \implies 2700+675x=2900+625x \implies 50x=200 \implies x=4\]