F.1 Maths 求解一題 PLZ

Let x be the number of apples bought ( note that's the same as the number of pears)
so
x apples at $6 each and $36 left means John has money of
6x + 36

x pears at $12 each and short of $18 means
12x - 18

Note that they are both equals the amount of money John has
so equating them gives

6x + 36 = 12x - 18
taking away 6x from both side gives
36 = 6x -18
addd 18 to both sides gives
54 = 6x
divided by 6 from both sides gives
x = 9

so John bought 9 apples.

2013-11-30 12:17:25 補充：
JEI YUEN LONG MATH & ENGLISH
JEI 元朗

John bought a certain number of apples at $6 each and he had $36 left.If he bought the same number of pears at $12 each instead of the apples , he would be short (不夠的) of $18. How many apples did John buy?


Let x be the total money of John
and y be the number of apples or pears.

x-6y=36 –(1)
x-12y=-18 –(2)

(1)-(2)：
6y=54
y=9

∴John bought 9 apples.

2013-11-30 00:10:50 補充：
題目話每個蘋果賣$6，John買咗＿個後，剩餘$36；
每個梨賣$12，如果買同樣數量的梨，就尚欠$18 (所以-18)

Let X be the total money of John
Let Y be the number of apples and pears

X - 6Y = 36 (1)
X - 12Y = 18 (2)

(1):
X - 6Y = 36
X - 6Y + 6Y = 36 + 6Y
X = 36 +6Y

sub (1) into (2):
X -12Y = 18
(36 + 6Y) - 12Y = 18
36 + 6Y -12Y = 18
36 - 6Y = 18
6Y = 18
Y = 3


X = 36 + 6Y
X = 36 + 18
X = 54

2013-11-29 23:43:11 補充：
Y = 3
SO , John buy 3 apples.