The proportion that represents the situation is:
1 pound of beef 2 pounds of beef
---------------------- = --------------------------
3 potatoes 6 potates
As you see, when you double thre recipe you multiply both numerator and denominator by 2: 1 pound of beef * 2 = 2 pounds of beef, 3 potatoes * 2 = 6 potatoes.
Of course, the fraction 1/3 is equivalent to the fraction 2/6.
