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\(\dfrac{30-x}{30}=\dfrac{8}{15}\)
\(\Rightarrow\left(30-x\right)15=30\times8\)
\(\Rightarrow\left(30-x\right)15=240\)
\(\Rightarrow30-x=240:15\)
\(\Rightarrow30-x=16\)
\(\Rightarrow x=30-16\)
\(\Rightarrow x=14\)
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\(\dfrac{x+30}{72}=\dfrac{5}{8}\)
\(\Rightarrow\left(x+30\right)8=5\times72\)
\(\Rightarrow\left(x+30\right)8=360\)
\(\Rightarrow x+30=360:8\)
\(\Rightarrow x+30=45\)
\(\Rightarrow x=45-30\)
\(\Rightarrow x=15\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(\dfrac{30-x}{30}\) = \(\dfrac{5}{15}\)
\(\dfrac{30-x}{30}\) = \(\dfrac{1}{3}\)
30 - \(x\) = \(\dfrac{1}{3}\) \(\times\) 30
30 - \(x\) = 10
\(x\) =30 - 10
\(x\) = 20
\(\dfrac{x+30}{72}\) = \(\dfrac{5}{8}\)
\(x+30\) = \(\dfrac{5}{8}\) \(\times\) 72
\(x+30\) = 45
\(x\) = 45 - 30
\(x\) = 15
![](https://rs.olm.vn/images/avt/0.png?1311)
a) 2/5+9/5=11/5
b) 15/45+25/30=7/6
c) 28/32+45/72=3/2
d) 8/28+5/30=19/42
bài 2
a) x - 4/5 =4/5 + (3/7+3/5)
x-4/5=4/5+36/35
x-4/5=64/35
x=64/35+4/5
x=92/35
b) x-7/35=4/5
x=4/5+7/35
x=1
nhớ tk m nhé
a = 18/5
b = 3/9 + 5/6 = 6/18 + 25/18 = 31/18
c = 7/8 + 5/8 = 12/8
d = 4/14 + 1/6 = 14/42 + 7/42 = 21/42 -= 7/14
b2
x - 4/5 = 4.5 + ( 3/7 + 3/5 )
x - 4/5 = 49/35
x = 49/35 + 4/5
x = 77/35
x - 7/35 = 4/5
x = 4/5 + 7/35
x = 1
mình ko chắc chắn đâu
mình nghĩ mình làm sai
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(\left(72.72+72.28\right)+x:8\)
\(\left[72.\left(72+28\right)\right]+x:8\)
\(\left(72.100\right)+x:8\)
\(7200+x:8\)
\(x:8=-7200\)
\(x=-57600\)
vậy \(x=-57600\)
![](https://rs.olm.vn/images/avt/0.png?1311)
![](https://rs.olm.vn/images/avt/0.png?1311)
Lời giải:
a.
$18\times 85+9\times 30=18\times 85+18\times 15=18\times (85+15)$
$=18\times 100=1800$
b.
$9\times 13+9\times 80+63=9\times 13+9\times 80+9\times 7$
$=9\times (13+80+7)=9\times 100=900$
c.
$8\times 11+16\times 40+72=8\times 11+8\times 80+8\times 9$
$=8\times (11+80+9)=8\times 100=800$
![](https://rs.olm.vn/images/avt/0.png?1311)
\(a.18\times85+9\times30\)
\(\text{=}1530+270\)
\(\text{=}1800\)
\(b.9\times13+9\times80+63\)
\(\text{=}117+720+63\)
\(\text{=}900\)
\(c.8\times11+16\times40+72\)
\(\text{=}88+640+72\)
\(\text{=}800\)
Ta có: