A school has 300 student, of whom 144 are boys.If 45% of the first year students are boys, while 50% of the other year students are boys, find the number of first year students in the school

That's how I would have done it:

Let f be the number of first year students and s be the number of second year students.

45% f + 50% s = 144

Then, taking into consideration the girls, which you have a total of 300 - 144, you have 156.

Then,

65% f + 50% s = 156

You have two equations, two variables. Solve simultaneously.

Hint: To have the value of f directly, eliminate the 50% s. Then, no need to find s.