$a)2345678\approx 2350000$

$187259,12\approx 190000$

$666666,6\approx 670000$

$b)6872,098\approx 6872$

$40013,809\approx 40014$

$15,4582\approx 15$

$c)352,1782\approx 352,2$

$0,208131\approx 0,2$

$0,995\approx 1$