i do 2U (not 3), but i agree that 10b is actually 2U work.... i know, its very very hard, i couldnt get it out... but i saw the answer to it from someone and its so damn simple when u think about it!!!!!!
they gave u the inequalities (x+y>15, x-y<15, or something like that), they ttried to trick people into believing it was a really hard PROBABILITY question, when in fact it was a CO-ORDINATE GEOMETRY question!!!! they gave u the inequalities, and the diagram (the square) so all u had to do was draw the lines y=15-x (taken from x+y=15) and y=x-15 (taken from x-y=15), solve the inequliaties (ie, test what side of the line its on) and shade the area between them. then find this area, which comes out to be a fraction, and thats ur probability!!!
(its a lot simpler on paper than trying to explain it) it takes about not even half a page to do..
then for the part (iii) u just work backwards, putting in P = 1/2 and find what t is...
it never comes to me in the exam tho, its always hours later that i realise! lol