Red Swastika School P5 CA2 2007 Math Question

Miss Chee bought some pencils, pens and notebooks. The number of notebooks she bought was 25% of the number of pens. The cost of a pencil, a pen and a notebook was $2, $3 and $7 respectively. The total cost of the pens and notebooks were $418 and the total cost of the pencils and notebooks were $262. How many pencils did Miss Chee buy?


Pencils (unknown no. of pencils) ----- ? x $2 each
Pens ----- 4 units x $3 each ---- 12 units (of dollars)
NB ----- 1 unit x $7 each ----- 7 units (of dollars)
("Pens" has been allocated 4 units while "NB" 1 unit because the number of NBs bought is 25% of the number of pens)


Pens + NB ----- $418
12 units + 7 units ---- $418
19 units ----- $418
1 unit ------ $418/19 = $22 (the all important 1 unit)

Pencils + NB ----- $262
? x $2 + 7 units ------ $262
? x $2 + (7 x $22) ----- $262
? x $2 + $154 ----- $262
? x $2 ----- $262 - $154 = $108
? ---- $108/$2 = 54

Answer: She bought 54 pencils