Brent has goldfish that quadruple (become four times as many) every month, and Gretel has goldfish that double every month.  If Brent has 4 goldfish at the same time that Gretel has 128 goldfish, then in how many months from that time will they have the same number of goldfish?
$\text{(A)}\ 4 \qquad \text{(B)}\ 5 \qquad \text{(C)}\ 6 \qquad \text{(D)}\ 7 \qquad \text{(E)}\ 8$
Call this month "Month 0".  Make a table of the fish that Brent and Gretel have each month.
$\text{Month / Brent / Gretel}$
$\text{0 / 4 / 128}$
$\text{1 / 16 / 256}$
$\text{2 / 64 / 512}$
$\text{3 / 256 / 1024}$
$\text{4 / 1024 / 2048}$
$\text{5 / 4096 / 4096}$
You could create a similar table without doing all of the calculations by converting all the goldfish into powers of $2$.  In this table, you could increase Brent's goldfish by two powers of $2$, while increasing Gretel's fish by one power of $2$.
Either way, in $5$ months they will have the same number of fish, giving an answer of $\boxed{B}$