Derrick received a total of 21 gift cards for graduation. He received gift cards in each of four denominations (sizes), $25, $50, $75, and $100. The number of $100 gift cards Derrick got was twice the number of $75 gift cards he received. The total value of the $50 gift cards he got exceeded (was above) the total combined value of $25, $75, and $100 gift cards by $100. How many $25 gift cards did Derrick get for graduation?