Nobody wants to do these because, I don't know why.
Answer:

Step-by-step explanation:
Let l litre of 7 grams of salt per litre is combined with x litres of water which contains y grams of salt per litre to yield a solution with 2 grams of salt per litre.
Thus,
Salt in l litre + salt in x litre = salt in resultant mixture
7l + xy = 2(l +x)
7l + xy = 2l + 2x
7l - 2l = 2x - xy
5l = x(2-y)

Hence,
litres of 7 gram of salt per litre is mixed.
Let the side of the square = a units, the area of the square = a^2 sq units
Area of the 4 semicircles = area of 2 circles = 2 pi r^2 = 2pi (1/2 a)^2
= pi a^2 / 2
Probability of a point in the unshaded square being picked) = area square / area of the total figure = a^2 / (a^2 * pi/2) = 1 / pi/2 = 2 /pi
which is approximately 65%