$3,1\cdot {10}^5=3,1\cdot 100000=310000$

$0,05\cdot {10}^3=0,05\cdot 1000=50$

$0,49\cdot {10}^4=0,49\cdot 10000=4900$

$0,008\cdot {10}^8=0,008\cdot 100000000=800000$