Suppose 15 of the people who voted for dee had not voted at all. How would this change affect the ratio of gene's votes to total votes

Gene's votes to Total votes

= Gene's votes: Total votes - 15

Since you did not provide the number of votes, I will give an example.

55 people voted for Dee, 45 people voted for Gene. This means the Total votes cast was 100.

The ratio of Dee's votes to total votes is: 55:100 ⇒ 11:20 when reduced

The ratio of Gene's votes to total votes is: 45:100 ⇒ 9:20 when reduced

If 15 people who voted for Dee did not vote, then the total number of voters is 85 so the ratios change as follows:

Dee: (55-15) : (100-15) ⇒ 40:85 ⇒ 8:17 when reduced

Gene: (45-0) : (100-15) ⇒ 45:85 ⇒ 9:17 when reduced