We have an unknown quantity, which is y. We are told that it varies joint as a and b. This translates as follows
where k is a constant.
Now, we are also told that it varies inversely as the square root of c. So this look as follows
where k is another constant. We have to combine both facts as follows
in this case, k is a constant. Now, we are told that whenever c=4, b=2 and a=6 we get y=24. So we get the following equation
By dividing by 6 on both sides, we get
So the general expression of y looks like this
Now, we want to calculate the value of y whenever a=3 and b=5 and c=25. So we get
so y=12 whenever a=3, b=5 and c=25.