Consider the mess charges for 35 students for 24 days is Rs. 6300. How much will be the mess charges for 25 students for 18 days.

Here , we have three quantities i.e mess charges, number of students and number of days.

Mess charges is directly proportional to number of students.

Mess charges $\infty$  number of students.

$6300:x=7:5$

Again mess charges are directly proportional to number of days.

Mess charges $\infty$  number of days.

$6300:x=4:3$

Since, mess charges depends upon both the value i. e number of students and number of days so we will take a compound ratio of these two variables.

Mess charges $\infty$  compound ratio of number of students and ratio of number of days.

$6300:x=$ compound ratio of $7:5and4:3$

$6300:x=7\times 4:5\times 3$

Product of means $=$  product of extremes.

$28\times x=15\times 6300$

$x=\frac{15\times 6300}{28}$

$x=Rs.3375.$

Hence, the required mess charges is $Rs.3375.$