Answer
$600$ strokes.
Work Step by Step
Let the required number of strokes is $x$.
Quantity of the fuel is proportional to the number of strokes.
We can write.
$\Rightarrow \frac{35}{420}=\frac{50}{x}$
Cross multiply.
$\Rightarrow 35\times x=50\times 420$
Divide both sides by $35$.
$\Rightarrow x=\frac{50\times 420}{35}$
Simplify.
$\Rightarrow x=600$ strokes.
