A school is given special funding according to the formula that there will be $300 given for each student along with a single $4000 grant. A

S = Number of students

$300 per student + single grant = Funding

300S + 4000 = 124 900

300S = 120 900

S = 120 900/300 = 403

Therefore the number of students is equal to 403

I would tend to use a formula if the equation were more complex.

$124 900 - $4000 equals $120 900

$120 900 divided by $300 equals 403

Therefore there are 403 students. 

(Because (403 X 300) + 4000 equals 124 900)

Very nice explanation anon 

you have solved it by going backwards.

Can you also do it by setting up an initial equation and solving the equation ?  

I like your answer but I am just setting you a challenge. 

Very nice anon - are you the same anon?