Let X = investment into Blue-ship   stocks (yield 10% return);

    Y = investment into growth      stocks (yield 15% return);

    Z = investment into speculative stocks (yield 20% return).


Then the objective function to MAXIMIZE is the total return 

    P(X,Y,Z) = 0.1X + 0.15Y + 0.2Z    (1)


The constraints are

    X + Y + Z <= 100000,          (2)    ("decided to invest at most N$100 000")

    Y + Z <= 0.3*(X + Y + Z)      (3)    ("no more than 30% of his investment should be in growth and speculative stocks")

    X + Z >= 0.5*(X + Y + Z)      (4)    ("at least 50% of his investment should be in blue chip and speculative stocks.")

    X >= 0,  Y >= 0,  Z >= 0.     (5)


The constraints  (3) and (4)  can be reduced to the standard form

    0.3X -0.7Y - 0.7Z >= 0        (3')  

    0.5X - 0.5Y + 0.5Z <= 0       (4')   


So, the set up is this set of equalities and inequalities  (1), (2), (3'), (4') and (5).


With this setup, find an appropriate solver and use it to get the numerical answer.