"increased by a factor of ..."

[divineacclivity]

Cost is expressed by the formula t.b^4. If b is doubled, by what factor has the cost increased?
(A) 2 (B) 6 (C) 8 (D) 16 (E) 1/2

So, if b is doubled, cost becomes 16 times as much.
So, the cost increased to 16 times the original cost.
Or, the cost increased by 15 times the original cost.
So, doesn't it translate to: "The cost has increased by a factor of 15" instead of "16" as in answer choice D???
Where am I wrong with this one? Please help. Thank you in advance.

Source: Manhattan's Equations, Inequalities, and VICs Guide 4th edition, Problem set of chapter - 4

-DA

[RonPurewal]

post105099.html

Please don't post the same thing in two different places. Thanks.