1 batch of muffins included 2/3 cups of raisins. We can use this to set a proportion: 1 : 2/3. That means for every 1 batch, she uses 2/3 cups of raisins.
Now, Ina made 2 1/2 batches. To find out how, we need to look at our proportion again: 1 : 2/3. Whatever operation you do to one side, you must do to the other. So, to get 2 1/2 batches, we need to multiply the left side of the proportion, which is 1 batch, by 2 1/2 to get 2 1/2 batches.
BUT, we also need to multiply the other side, 2/3 cups, by 2 1/2. We can start by converting this into an improper fraction, 5/2. (Because 1=2/2, so 2=4/2, and 4/2+1/2=5/2.) When multiplying fractions, you multiply the numerators together to get the new numerator, and the denominators to get the new denominator. 2*5=10, and 3*2=6. Our new fraction is 10/6. Since these are both divisible by 2, we simplify by dividing each side by 2 to get 5/3. Finally, we can convert this into a mixed number: 1 2/3. (Because 1=3/3 and 5/3 - 3/3 = 2/3, and 1 + 2/3 = 1 2/3.) This is the cups of raisins she used.
Answer: She used 1 2/3 cups of raisins.