Digits are 4,5,6,7,8 for number to be greater than 600 at the hundred place 6,7,8 appear and this can be done 3 ways .
for remaining tow places we choose two number from remaining four digits. this can be done 4C2 ways = 4!/(2!2!) = 6 and two number can be interchanged by 2! = 2 ways .
so total numbers which is greater than 600 are = 2*3*6 = 36.
if you can not understand please ask.