The ratio of boys to girls in mr castello's class is 2 to 3 which of the following cannot be the total number of students in mr castello's class 20 24 25 30

A ratio of 2:3, simply means, you divide some "total" by 2+3, or 5, and then to the ratio of "2" section, you give 2 of those quotients, and to the ratio "3" section, you  give it 3 of those.

now, let's check for example 20, 20/5 is 4, 4*2 is 8 boys and 4*3 is 12 girls.

let's check 25, 25/5 is 5, so 5*2 is 10 boys, and 5*3 is 15 girls.

let's check 30, 30/5 is 6, so 6*2 is 12 boys, and 6*3 is 18 girls.

so those work.  now let's check 24.

24/5 is 4.8,   4.8*2 is 9.6 boys, so 9 boys and 0.6 of another, so we have to chop off an arm or something.

and 4.8*3 or 14.4 girls, so 14 girls and 0.4 of another, so, we have to chop off both legs and arms or thereabouts.

anyhow...so you'd see which one doesn't work and why.