Hey guys, the question is 6.b. in the picture : http://imgur.com/FaKUMUZ Here is what I did to solve it : http://imgur.com/YrIvbTO I made these two simultaneous equations. 1875 comes from the fact that U1 + U2 = 1500 and U3 + U4 = 375. Then S4 must equal 1500+ 375(1875). I then found a formula for 'a' and substituted it back into S2, which I further simplified into a polynomial of degree 3 and I got the solutions r = 1, .5 & -.5. r can't be 1 as then the whole series would stay at whatever 'a' is, so then I concluded that it must be .5 and -.5. I then substituted .5 and -.5 into one of the equations and solved for 'a' which is 1000 & 3000 respectively. That is the actual answer, but it seemed a bit long-winded for 4 marks. Is there an easier way to do it? Any help would be appreciated.