One plastic robot is given away free inside each packet of a certain brand of biscuits. There are four colours of plastic robot (red, yellow, blue and green) and each colour is equally likely to occur. Nick buys some packets of these biscuits. Find the probability that 

i. He gets a green robot on opening his first packet,

ii. He gets his first green robot on opening his fifth packet.

Nick’s friend Amos is also collecting robots.

iii. Find the probability that the first four packets Amos opens all contain different coloured robots.