$a)46\approx 50$

$181\approx 180$

$310908\approx 310910$

$2013\approx 2010$

$1099\approx 1100$

 

$b)3124\approx 3100$

$1080\approx 1100$

$94\approx 100$

$1560\approx 1600$

$45\approx 0$

 

$c)4321\approx 4000$

$654321\approx 654000$

$1687\approx 2000$

$36800\approx 37000$

$12987\approx 13000$