The profit percentage formula is (selling price - cost)/cost * 100 = profit percentage

Let $s$ = initial selling price, $c$ = commodity's cost.

Since we're given the initial profit percentage is 30%,
$\frac{s - c}{c} * 100 = 30$
$s - c = 0.3c$
$s = 1.3c$

The new selling price after the increase = s + 100. Set up the formula again with the new profit percentage of 40%.
$\frac{(s + 100)- c}{c} * 100 = 40$ <- c remains the same, since cost doesn't change
$(s + 100)- c = 0.4c$
$s + 100 = 1.4c$

Plug in $s = 1.3c$ for s to solve for c.
$1.3c + 100 = 1.4c$
$100 = 0.1c$
$c = 1000$

$s = 1.3c = 1.3(1000) = 1300$