two questions about maths

1) 5(f-0.4) = 2.2-9(0.2f+1.6)
5f-2 = 2.2-1.8f-14.4
5f+1.8f = 2.2-14.4+2
6.8f = -10.2
f=-1.5

2014-10-04 15:28:16 補充：
17+n=3〔2(7-4n)+5n〕
17+n = 3(14-8n+5n)
17+n = 42-24n+15n
10n =25
n=2.5

2014-10-04 15:37:24 補充：
就是把同樣的數放在同一邊!
轉位要轉正負號!!

1)

5(f - 0.4) = 2.2 - 9(0.2f + 1.6)

STEP 1: expand

5f - 5(0.4) = 2.2 - 9(0.2f) + (-9)(1.6)
5f - 2 = 2.2 - 1.8f -14.4
5f - 2 = -1.8f -12.2

STEP 2: REARRANGE and put all the 'f' to one side

5f + 1.8f = -12.2 + 2
6.8f = -10.2
f = -10.2 ÷ 6.8
f = -1.5

2)
17 + n = 3[ 2(7-4n) + 5n ]

STEP 1: expand:

17 + n = 3 [ 14 - 8n + 5n ]
17 + n = 3 (14 - 3n)
17 + n = 42 - 9n

STEP 2: REARRANGE and put all the 'n' to one side

n + 17 = -9n + 42
n + 9n = 42 - 17
10n = 25
n = 2.5