Answer
The speed of the truck relative to the ground is $30.0~m/s$
Work Step by Step
Let $v_{b,g}$ be the velocity of the ball relative to the ground.
Let $v_{b,t}$ be the velocity of the ball relative to the truck.
Let $v_{t,g}$ be the velocity of the truck relative to the ground.
We can find the velocity of the truck relative to the round:
$v_{b,g} = v_{b,t}+ v_{t,g}$
$v_{t,g} = v_{b,g} - v_{b,t}$
$v_{t,g} = (-10.0~m/s) - (-40.0~m/s)$
$v_{t,g} = 30.0~m/s$
The speed of the truck relative to the ground is $30.0~m/s$
