兩數的和是120。如果從較大的數中減去10,則所得的差是較小的四倍。求該兩數。

Let X > Y

X + Y = 120 -- (1)
X - 10 = 4Y
X = 4Y + 10 -- (2)

put (2) in (1):

4Y + 10 + Y = 120
5Y = 110
Y = 22

X = 4(22) + 10
X = 98

Let X > Y

X + Y = 120 -- (1)
X - 10 = 4Y
X = 4Y + 10 -- (2)

put (2) in (1):

4Y + 10 + Y = 120
5Y = 110
Y = 22

X = 4(22) + 10
X = 98

╭ x + y = 120 --------------------------------[1]
╰ y = ( x - 10 ) ÷ 4 -------------------------[2]

從[1]，
y = 120 - x --------------------------------------[3]

把[3]代入[2]，
.120 - x = ( x - 10 ) ÷ 4
480 - 4x = x - 10
. 490 = 5x
. x = 98

把 x = 98 代入[1]
98 + y = 120
. y = 22

2006-12-23 13:08:40 補充：
x 是大數y 則是細的那個

假設兩數是 a & b
a + b = 120 ---- (1)
a-10 = b x 4-----(2)

in (1) a + b = 120
a = 120 - b --- 代入 (2)

120 - b - 10 = 4b
110 - b = 4b
110 = 4b+b
110 = 5b
b = 22

a = 120-22 = 98