Answer
$\$1800$
Work Step by Step
Original number of students $=3$
Original cost per student $=\frac{x}{3}$
Original cost per rent $=x$
The formula is, number of student $\times$ cost per student $=$ cost of rent.
After fourth student is added,
Number of student becomes $=4$
Cost per student $=\frac{x}{3}-150$
Cost of rent $=x$
Therefore, applying the formula,
$(4)\left(\frac{x}{3}-150\right)=x$
$\frac{4x}{3}-600=x$
$\frac{4x}{3}-x=600$,
$\frac{x}{3}=600$
$x=1800$
