數學 方程式(急help me><)麻煩各位識幾多做幾多><

1)
4個 x $5 = $20, 1個 x $2 = $2, 7個 x $1 = $7,
合計 = $20 + $2 +$7 = $29, 12個硬幣
$29 - $20 = $9

3個 x $5 = $15, 2個 x $2 = $4, 7個 x $1 = $7,
合計 = $15 + $4 +$7 = $26, 12個硬幣
$26 - $20 = $6

2個 x $5 = $10, 3個 x $2 = $6, 7個 x $1 = $7
合計 = $10 + $6 +$7 = $23, 12個硬幣
$23 - $20 = $3

硬幣的組合有三可能性:
4個5美元, 1個2美元, 7個1美元
3個5美元, 2個2美元, 7個1美元
2個5美元, 3個2美元, 7個1美元

2)從51減去一個數的三倍的結果是30。現在n是數量,求找出n的數量。

51 - 3 n = 30
51= 30 + 3n
51 - 30 = 3 n
7 = n
n = 7 (答案)

3) 從一個正方形周邊的18減法的結果是26。找出每個正方形的長度
設每個正方形的長度是y:
4 y - 18 = 26
4 y = 44
y = 11 (答案)

4) 如果兩個連續偶數的總和是86，請找出較小的數字。
(原文: if the sum of two consecutive even numbers is 86, find the smaller number.)

Let “y” be the smaller number

The larger number is y + 2

Equation: y + (y + 2) = 86
2y + 2 = 86
y = 86 – 2
2y = 84
y = 42
較小的數字是 42 (答案)