Together they eat 2/3 pizza in 4 seconds -----------> hence, together they eat the entire pizza in 6 seconds.


Let x be the time for Dien in seconds to eat the pizza alone.


Then the time for Jane to eat the pizza alone is (x-5) seconds.


The rate of eating is    for Dien  and    for Jane in terms of entire pizza per second.


The equation for the combined rate is


     +  = .


To solve it, multiply both sides by  6x*(x-5).   You will get then


    6*(x-5)  + 6x = x*(x-5)


Reduce to the standard quadratic equation


    x^2 - 5x - 12x + 30 = 0

    x^2 - 17x + 30 = 0


Factor left side


    (x-15)*(x-2) = 0.


The only positive root x= 15 seconds for Dien provides the positive value (x-5) = 10 seconds for Jane.


ANSWER.  15 seconds for Dien and 15-5 = 10 seconds for Jane to eat the entire pizza alone.