math
posted by keisha on .
Marcus has 68 ftof fencing. He wants to build a rectangular pen with the largest possible area. What should the dimensions of the rectangle be?
a. 19 ft by 21 ft
b. 21 ft by 13 ft
c. 17 ft by 17 ft
d. 19 ft by 15 ft
I got a because I multiplied 19 x 21 and I got 399. Is this right?

First, you have to check to make sure you have sufficient fencing. Take each of the four options and calculate the perimeter to make sure you have no more than 68 ft.
For the first one, the perimeter would be 19+19+21+21 or 19(2) + 21(2), both of which equal 80. You don't have enough fencing for that, so eliminate the option.
Then, with your remaining options, calculate the area by multiplying the sides as you did above to see which is largest. 
Is it c. 17 by 17?

Yes, that's right.
17 + 17 + 17 + 17 = 68
or
17(4) = 68