Answer
The brain was growing at a rate of $~~1.05\times 10^{-8}~g/year$
Work Step by Step
$W = 0.12~L^{2.53}$
We can find an expression for $B$:
$B = 0.007~W^{2/3}$
$B = 0.007~(0.12~L^{2.53})^{2/3}$
$B = 0.001703~L^{1.687}$
We can find $\frac{dL}{dt}$:
$\frac{dL}{dt} = \frac{20~cm-15~cm}{10\times 10^6~years}$
$\frac{dL}{dt} = 0.5 \times 10^{-6}~cm/year$
We can find $\frac{dB}{dt}$:
$B = 0.001703~L^{1.687}$
$\frac{dB}{dt} = (0.001703)~(1.687~L^{0.687})(\frac{dL}{dt})$
$\frac{dB}{dt} = (0.001703)~(1.687)~(18)^{0.687}(0.5 \times 10^{-6}~cm/year)$
$\frac{dB}{dt} = 0.0105\times 10^{-6}~g/year$
$\frac{dB}{dt} = 1.05\times 10^{-8}~g/year$
The brain was growing at a rate of $~~1.05\times 10^{-8}~g/year$
