I'm not very good with recurrence relations ... so I guess the only way to get better is practice.
This is the qn I'm now having problems doing:
Suppose that we have 4 cent stamps, 6 cent stamps, and 10 cent stamps, each in unlimited supply. Let f(n) be the number of ways of obtaining n cents of postage if the order in which we put on stamps counts. For example, f(4)=1 and f(8)=1 (two 4 cent stamps), while f(10)=3 (one 10 cent stamp, or a 4 cent stamp followed by a 6 cents stamp, or a 6 cent stamp followed by a 4 cent stamp).
(a) If n >10, derive a recurrence for f(n). (b) Use the recurrence of part a) to find the # of ways of obtaining 14 cents of postage. (... this is just f(14) right?)