Circular pen means you are going to use Area = Pi(r^2).
So 78.5 = pi(r^2). Divide by pi on both sides to get r^2 = 78.5/pi. Take the square root of that to get r or radius = approximately 4.99. If you need an exact answer then use

for the next part because you are not done yet.
Plug whatever R you use into the circumference formula of Circumference = 2(pi)(r).
If you use 4.99, you do 2(pi)(4.99) which is approximately 31.35. If you use the exact answer, you do 2(pi)(

) which gives you approximately 31.41.
Either way you'll need about 31 meters of chicken wire but the 31.40796 or ~31.41 is closer to the exact answer.
For perfect solution we need to have :- 8 colves in 500ml solution
500 ml has 8 cloves
1 ml has 8 / 500 cloves
100ml has 8 / 500 * 100 = 8/5 = 1.6 cloves
Raphael's mixture
900 ml has 12 cloves
1 ml has 12 / 900 cloves
100 ml has 12 / 900 * 100 = 12 / 9 = 1.33 cloves
so concentration of garlic in 100 ml solution of Raphael's solution is less than Emily's solution so it is not garlicky enough. ( option B)
The answer would be: X=26