600 round to the nearest tenth is 600 because there is no tenth place. Well there is but its a 0. If the number was 658 the answer would be 660.
A rational expression is undefined if the denominator is zero. Since the denominator is
, the expression is undefined if

the answer is 4 because 16 and 20 can both be divided by 4.
8x=6x+20
Btw I figures it out x=10