Mrs. Gifford's class consists of 15 girls and 25 boys.

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To find the mean of the boy's scores, we need to first find the total scores of all the boys and then divide it by the number of boys.

Given that the class mean was 80 and the mean girl's score was 75, we can find the total scores of all the students in the class by multiplying the class mean (80) by the total number of students (40, which is 15 girls + 25 boys).

Total scores of all students = Class mean * Total number of students
Total scores of all students = 80 * 40
Total scores of all students = 3200

Now, we need to subtract the total scores of the girls from the total scores of all the students to get the total scores of the boys.

Total scores of the boys = Total scores of all students - Total scores of the girls
Total scores of the boys = 3200 - (75 * 15)
Total scores of the boys = 3200 - 1125
Total scores of the boys = 2075

Finally, to find the mean of the boy's scores, we divide the total scores of the boys by the number of boys.

Mean of the boy's scores = Total scores of the boys / Number of boys
Mean of the boy's scores = 2075 / 25
Mean of the boy's scores = 83

Therefore, the mean of the boy's scores is 83.