just use what you know about this stuff
(a+36d)/(a+20d) = (a+55d)/(a+36d)
(a+36d)^2 = (a+55d)(a+20d)
a^2+72ad+1296d^2 = a^2+75ad+1100d^2
3ad = 196d^2
3a = 196d
That is, for any value of n,
a=196n
d=3n
So, there is no unique solution.
If n=1, then a=196 and d=3. The terms are
196+20*3 = 256
196+36*3 = 304
196+55*3 = 361
304/256 = 361/304
You can easily verify that it works for any value of n.
