Question 1781
with this type of question, the hardest bit is converting the english into maths, so we have to step through it carefully.

First, lets define a couple of things we are looking for:

Let x = number of large font pages.
let y = number of small font pages.

Look at the english...he uses a number of large font pages and some small font pages. The total number of words is 48000.

So...how many large font words does he use? Well, 

on 1 page...    1800
on 2 pages... 2*1800
on 3 pages... 3*1800
so
on x pages... x*1800 --> we write this as 1800x

Similarly, how many small font words does he use? Well, again...

on 1 page...    2400
on 2 pages... 2*2400
on 3 pages... 3*2400
so
on y pages... y*2400 --> we write this as 2400y

So, the total has to be 48000, ie

{{{(1800x) + (2400y) = 48000}}}. This is equation 1.

Now we need another equation. The only other bit of info is that the total number of pages = 21.

Well, we know he uses x large print pages and y small print pages, so:

{{{x + y = 21}}} is equation 2

Now you have 2 equations, 2 unknowns...you can solve them simultaneously. Have a go. Answer is x=4, y=17


cheers
Jon.