Start by fixing both chart totals from the two clues given. Company C's 21% share of Fertilizer-P equals 2520 Kg, which gives total Fertilizer-P as $2520/0.21 = 12000$ Kg. Company B's 21% share of Fertilizer-Q equals 3150 Kg, which gives total Fertilizer-Q as $3150/0.21 = 15000$ Kg. Company C's share in the Fertilizer-Q chart is 30%, so its actual sale is $0.30 \times 15000 = 4500$ Kg. Comparing this against the Fertilizer-P total: $\frac{4500}{12000} \times 100 = 37.5$. \[\boxed{37.5}\]