A green liquid is mixed with a blue liquid in the proportion 4 : 5 and the mixture is sold at Rs. 20 per litre at a 20% profit. If the green liquid costs Rs. 4 more per litre than the blue liquid, what does the green liquid cost per litre approximately.

Let the cost of blue liquid per litre be $$'x'$$

then the cost of green liquid per litre is $$'x+4'$$

The S.P. of the mixture is Rs 20 per litre, and the profit % is 20%.

Then C.P. of the mixture per litre = $$\frac{20}{1+0.2}=\frac{20}{1.2}=\frac{50}{3}$$......(1)

Also, the C.P. of the mixture per litre is given by = $$\frac{4\left(x+4\right)+5\left(x\right)}{5+4}=\frac{9x+16}{9}$$.....(2)

Equating (1) to (2)

we get $$\frac{50}{3}=\frac{9x+16}{9}$$

$$150=9x+16$$

$$9x=134$$

$$x=14.88$$

Hence, the cost of green liquid per litre is = 14.88+4= 18.88.

Option (B) is correct.