Tim said:

It's not a matter of "making up" the value movements,

It is if I use "make up" in the sense of "posit ficticious

values for". Naturally it goes without saying that you can't

make up *arbitrary* values for f and g, the two years'

inflation factors. There is only one pair of values which

will work to achieve the effect you wanted, and of course it

must be calculated.

but rather *calculating* them from their yield.

Well, I suppose you can do it by working out the yields first, but

that seems a bit long-winded. How I would do it is to observe that

the two three-way ratios 100fg:60g:75 and 20:15:17 must be

equal, which boils down to solving the three equations

[1] 75k = 17

[2] 60gk = 15

[3] 100fgk = 20

for f and g, which *can* be done by calculating k first (which

happens to be one of the yields) from [1], then using that in [2]

to work out g, and finally using that in [3] to work out k, but it

can also be by eliminating k algebraically, calculating g directly

by dividing [2] by [1], and f by dividing [3] by [2].

60g/75 = 15/17 => g = 1.103

100f/60 = 20/15 => f = 0.8

Of course, but in the absence of further information (the question

had no other data), it's got to be the best assumption, hasn't it?

It's the only possible assumption, which makes it simultaneously the

best and the worst. But since it is objectively bad, might it not be

better than solving the problem on the basis of it, to throw up one's

hands in warning and say that the answer will be silly so there is no

point in working it out?