Question 40139
The area of the barn is: 15*25 = 375 squared meters
The area of the stretch of rope is:
{{{A = (pi)r^2/4}}} because it is a fourth of the area of a circle
a) 10 meters long?
{{{A = (pi)100/4}}}
{{{A = (pi)25}}} about 78.54 squared meters
I think you can do the rest with the formula presented.
e) Zoe is happiest when she has at least 400 m2 to graze. What possible lengths of rope could be used? 
{{{400 <= (pi)r^2/4}}}
{{{1600/(pi) <= r^2}}}
{{{sqrt(1600/(pi)) <= r}}}
{{{22.57 <= r}}}