If we let x be the speed of the boat you are taking to the island (the steamboat), then we know that the speed of the other boat (the tall ship) is 10 knots less than the steamboat's. So the speed of the tall ship (for the return trip) is (x - 10) knots.
Jessica solved for the correct value of a, but Sally's review was incorrect.
Jeremy's total monthly costs amount to
$1500 + 6000 = $7500
Jeremy gets revenue of
(12×$3)×(1 - 0.10) = $32.40
per box of chocolates sold.
In order to cover costs, Jeremy must sell each month
$7500/($32.40/box) = 232 boxes . . . . . . (rounded up so costs are covered)