Question 137772
You're on the right track.
You set up the ratio correctly.
Thinks of it as 6 rabbits is to 10 days, like x rabbits is to 25 days. 
{{{6/10=x/25}}}
If we multiply both sides by 25 (the multiplicative inverse of 1/25), you get
{{{25*(6/10)=25*(x/25)}}}
{{{150/10=cross(25)*(x/cross(25))}}}
{{{x=15}}}
15 rabbits
You could have also cross multiplied,
{{{6/10=x/25}}}
{{{10x=6*25}}}
{{{10x=150}}}
{{{x=15}}}
Same result.