[100] * (1.3 ^ ((28 - 55)/10))) =

I assume the * means multiply?

= 28 - 55 = -27

[100] * (1.3 ^ (-27/10)))

= -27/10 = -2.7

[100] * (1.3 ^ (-2.7)))

= 1.3 ^ -2.7 (^ = to the power of) = 0.49243957607

[100] * (0.49243957607)

= 100 x 0.49243957607 = 49.243957607

So:

[100] * (1.3 ^ ((28 - 55)/10))) = 49.243957607