Answer
See below.
Work Step by Step
If the variables vary directly, then $T=aV$ for some constant $a$. Plugging in the given values, here $300=a\cdot4.8\\a=62.5$.
Thus the equation is: $T=62.5V$.
Thus if $T=420$, then $420=62.5V\\V=\frac{420}{62.5}=6.72$
