Exploring Mathematics F2 問題 ( 星期一要交功課喇 ).

let x be Correct
let y be Wrong
let z be No respons

then eqt 1) x+y+z=10
eqt 2) 20x-10y-5z=25

from 1) z=10-x-y
sub into eqt 2)
we have 20x-10y-5(10-x-y)=25
=> 25x-5y=75

therefore we have two solutions because (according to eqt 1.) x<=1 and x>=5 are impossible

x=3, y=0, z=7
and
x=4, y=5, z=1