Two mixtures have the same percentage of an acid solution. The first mixture has 7 fluid ounces of solution with xfluid ounces of water. The second mixture has 3æ fluid ounces of solution with 21 fluid ounces of water. The equation 7/x = 3x/21 represents this situation. How many fluid ounces of water are in the first mixture?
a. 7 fluid ounces
b. 7 or -7 fluid ounces
c. 21 or −21 fluid ounces
d. 21 ounces