Answer
$a_{block}~~$ can range between $~~0~~$ and $~~5.0~m/s^2$
Work Step by Step
The smallest value for $a_{block}$ would occur when only the slab accelerates because the block-slab interface is frictionless. In this case, $a_{block} = 0$
The largest value for $a_{block}$ would occur when the slab and the block accelerate together:
$F = (10~kg+10~kg)~a_{block}$
$a_{block} = \frac{F}{10~kg+10~kg}$
$a_{block} = \frac{100~N}{20~kg}$
$a_{block} = 5.0~m/s^2$
$a_{block}~~$ can range between $~~0~~$ and $~~5.0~m/s^2$
