Cost is expressed by the formula tb^4

Cost is expressed by the formula tb^4. If b is doubled, the new cost is what percent of the original cost?

(A) 200
(B) 600
(C) 800
(D) 1600
(E) 50

I read the explanation in Manhattan but cant relate to it.

I worked it out this way

Assume,
t=1
b=2

Original = (1*2)^4= 16
New= (1*2*2)^4 = 256

% change= (change/Original)*100
=(240/16)*100
=1500

Where am I going wrong?

Thanks