\(\because b\propto a^3\, \, \therefore b=ka^3\) [\(k\) is a nonzero constant]
\(\because a\) increases in the ratio 2:3.
So, let the initial value of \(a\) be \(2m\) and the increased value be \(3m\).
\(\therefore\) Before the increase, \(b=k\cdot (2m)^3 = 8km^3\)
And after the increase, \(b=k\cdot (3m)^3=27km^3\)
\(\therefore\) The ratio of increase in \(b\) is \(\cfrac{8km^3}{27km^3}=8:27\)
\(\therefore\) \(b\) will increase in the ratio 8:27.