Question 1884
the thing we want is the number of students, so let us define that x = number of ORIGINAL students.

so, originally, the cost per person, C = 240/x

With 4 students then dropping out, we have a new cost per person, C+2 = 240/(x-4).

Equating these 2 equations for C (ie get C= in both equations), we get that

{{{((240)/(x-4)) - ((240)/(x)) = 2}}}

multiplying both sides by x(x-4), to get rid of the denominators, leaves:

{{{240x - 240(x-4) = 2x(x-4)}}}

Multiplying out the brackets and simplyfying gives:

{{{240x - 240x + 960 = 2x^2 - 8x}}}
{{{x^2 - 4x - 480 = 0}}}

We can factorise this as (x+20)(x-24) = 0

Hence x=-20 or x=24. So our answer is x=24.

However, this is the original number of students (see the definition i gave of x. Hence there were 20 students who went to the museum.

cheers
Jon.