关于mathematica画图的基本问题!

关于mathematica画图的基本问题!
式子
dn/dt=r n(1-n/k)-(m (n/a)^2)/(1+(n/a)^2)
a)
On one graph,plot the new predation term (2nd term on right hand side of equation (2)) between n = 0 and n = 100 with m = 5 and a = 1,10,20,50.Remember to label your axes and include a descriptive legend,and make sure the whole axis is shown.
b)
Solve equation (2) numerically between t = 0 and t = 10,using the initial condition n = 1 at t = 0 and the parameter values r = 0.1,k = 100,m = 5 and a = 20.Plot your solution on axes between n = 0 and n = 10,and with appropriate labels.