The first equation has to be the sum of hotdogs and hamburgers. Since a total of 300 hamburgers and hotdogs was sold, the first equation is:
h+d=300
Now, the second equation is about the money. They say that EACH hamburger cost $2 and each hotdog cost $1. And, the total amount of money made was $420. So the second equation is:
2h+d=420
SO, overall, the system of equations is this:
h+d=300
2h+d=420
(option A is the answer)