Therefore, the postal code is..........

61425*5=307125, as 1750 divided by 5 is 350. Thus, (350th term)*5=postal code!

So smart of me!!!!

in the end part (0.5)(350)(350 +1) also uses AP concept mah....

now i appreciate the power of the GC (shielding power), in the past i am worried once such question approaches me like a devil.

I still remembered the days where parents lamented SEAB for this PSLE question

1+ 2+3+4+ ...+100

Nice olden days

But for EMaths students without such heavy armament, Number Patterns, like its Plane Geometry counterpart in AMaths, usually requires a little 阴阳眼 prowess in order to 'see' the pattern and unravel the Da Vinci Code-styled number sequences, though there are certain standard formulae for common sequences.

In this case there are many ways to get the simple answer, as described by clarion and TJK, but some students will have the tendency to stare at the sequence for a long, long time and wonder how on earth to 'minus every 1st-4th terms from each set of five terms' (or whatever) to obtain your 5+10+15+...+1750.

In times like these, if you ever find yourself going deeper and deeper into a pit in the midst of your exam, it's always good to prevent a 走火入魔 situation by either

1) revisit this part later (esp if it carries little marks) or
2) take a deep breath, look up to the heavens, invoke the changing your point of view strategy described in your textbook OPEN your eyes, like clarion with his 阴阳眼 has done to get:

5 + 10 + 15 + ... + 1750
= 5(1 + 2 + 3 + ... + 350)
= 5[(0.5)(350)(350+1)] = 307125
Note: This is a hence question so you must use T₃₅₀ to obtain the final answer.

So sweet, so simple - even the Ah Beng's primary school-educated henchmen should have no problem with this (without the need for frantic pressing of calculators) ... such is simplicity of most Number Patterns questions, since afterall it's a Sec One topic!

Sorry i can't impress you with nice mathematical equations, i am simply too lazy.

Using primary school rudimentary method:

First time plus last term : 5 + 1750 = 1755
Number of terms: ((1750-5) / 5) + 1 = 350
Total sum:( 350 X 1755) / 2 = 307 125

Using frantically pressing of P 6 calculator : 307 125

(Or) a smart way to use TI-84 Plus: 307 125

A level method:

1st term: 5
last term: 1750
d = 2
n = 350
Sn = 350 /2 ( 2 (5) + ( 350 - 1)( 5) ) = 350/2 (5 + 1750)
= 307 125

AP is so fun =)

Pity those people that are sabotaged by SEAB years ago by this equation
1 + 2 + 3 + 4 + ...+ 99 + 100

(Sum of AP, just using non-A Level stuff to solve.)

5+10+15+...+1750=5(1+2+3+4...+350)
=5x61425
=307125