Solution 1: Let 10% and 18% saline be x kg and y kg respectively.

x+y=300， 10% x+ 18% y = 300 * 15%

Solution: x = 1 12.5, y = 187.5.

Solution 2: Let the concentrations of the two solutions be x% and y% respectively.

Then: x+y=2*87, 5x+7y= 12*87.5.

Solution: x=84, y=90.

Then the concentrations of these two solutions are 84% and 90% respectively.