Question 825088
The q#1 is a simple two-variable system of equation exercise.  The #2 and #3?  Maybe not.


#1:


We want variables for price of oranges and price of apples.  Pounds(money unit) per kilogram.
x = apple price
y = orange price


{{{12x+24y=2160}}} and {{{24x+12y=1800}}}
'
{{{x+2y=180}}} and {{{2x+y=150}}}


{{{2x+4y=360}}}, and {{{2x+y=150}}} so subtract the second equation from the first equation.
{{{(2x+4y)-(2x+y)360-150}}}
{{{3y=210}}}
{{{highlight(y=70)}}}, pounds per kilogram oranges.
{{{x=180-2y=180-2(70)=highlight(40)}}} , pounds per kilogram apples.
'
Use these x and y to answer your question.


#3:

You are not clear about, "student" and "first student".  You might mean, a student scored 5x and x+10.  The only student who scored better received 118 for marks.  This must mean that the 118 marks is greater than the 5x+x+10 marks.


{{{5x+x+10<118}}}, and naturally, {{{x>0}}};
{{{6x<108}}}, and {{{x>0}}};
{{{highlight_green(0<x<18)}}}
As a practical feel, I'd imagine x must really be 1 or more, so a better choice about x is to say:
{{{highlight(1<=x<18)}}};  but that choice may be just arbitrary.