Write the ratio as a fraction in lowest terms.

1 2/3 to 3 1/3

First, let's convert the mixed numbers to improper fractions.

1 2/3 is a mixed number that can be converted to an improper fraction as follows:

1 2/3 = (1 * 3 + 2) / 3
= (3 + 2) / 3
= 5/3

3 1/3 is another mixed number that can be converted to an improper fraction:

3 1/3 = (3 * 3 + 1) / 3
= (9 + 1) / 3
= 10/3

Now, we need to write the ratio 1 2/3 to 3 1/3 as a fraction, using the improper fractions:

(5/3) / (10/3) = (5/3) * (3/10) (This is because dividing by a fraction is the same as multiplying by its reciprocal.)

Now, simplify the fraction:

= 5/3 * 3/10
= (5 * 3) / (3 * 10)
= 15 / 30
= 1 / 2 (This is after dividing both the numerator and the denominator by their greatest common divisor, which is 15 in this case.)

Therefore, the ratio 1 2/3 to 3 1/3 as a fraction in lowest terms is 1/2.