Correct Answer: Only I

Solution : Statement I = Average of all demands = 420
⇒ Sum of demands = 400 + 450 + 250 + 150 + 850 = 2100
$\therefore$ Average = $\frac{2100}{5}$ = 420
$\therefore$ Statement I is correct.
Statement II = The production of P and Q together is 52.60 percent of the production of S and T together.
⇒ Production of P and Q together = 500 + 650 = 1150
⇒ Production of S and T together = 850 + 1050 = 1900
$\therefore$ Percentage = $\frac{1150}{1900}\times100=60.5\%$
$\therefore$ Statement II is incorrect
Hence, the correct answer is Only I.