So say, we add "x" of the 60% saline solution, now, how much salt is in that solution? now 60% is salt but the rest is something else, but only salt is 60% of "x", or (60/100) * x, or
0.6x.
likewise, for the 10 Liters, if only 50% is salt, then is just (50/100) * 10 or
5.
now... let's check the water whilst we add "y" amount, the water has no salt at all, freshwater for that matter, so, salinity is 0%. or (0/100) * y or
0.

The answer is B, 57 degrees. You can tell angle C is 57 degrees since 180-123 is 57 and both angles add up to 180 on a straight line. Then since the lines are parallel, angle a is 55 degrees because of alternate interior angle theorem. Then, a triangle’s angles add up to 180 so to figure out angle d, you do: 180- (55 + 57) which gives you 68. Since 55 + angle b + angle d = 180, you have to do 180 - (68+55) = 57 degrees. Hope this helps!