Let original cost price = k. Then selling price at 25% profit = 1.25k.
If he had bought at 20% less of the cost price, i.e.,at 80% of cost price, then his cost price would have been = 0.8k.
Then selling price would be 10.50 less than original selling price 1.25k, his selling price would have been = 1.25k - 10.50.
Therefore his profit would have been = SP - CP = (1.25k - 10.5) - 0.8k
= 0.45k - 10.5.
This profit of (0.45k - 10.5) is on the cost price 0.8k.
Hence profit % = (Profit / CP) x 100
= [(0.45k - 10.5) / 0.8k] x 100 = 30
or 0.21k = 10.5 ==> k = 10.5 / 0.21 = 50