Answer
The watermelon is moving at a speed of 70 m/s when it passes Superman.
Work Step by Step
The watermelon and Superman will have covered the same distance when the watermelon passes Superman.
$d_{watermelon} = d_{super}$
$\frac{v+v_0}{2}~t = v_{super}~t$
$v = 2~v_{super}$
$v = (2)(35~m/s)$
$v = 70~m/s$
The watermelon is moving at a speed of 70 m/s when it passes Superman.
