30-[4.(x-2)+15=3

[4.(x-2)+15]=30 - 3 

[4.(x-2)+15]= 27 

4.(x-2) = 27 - 15 

4.(x-2) = 12

x - 2 = 12 : 4 

x - 2 = 3 

x = 3 + 2 

x = 5 

nha bạn 

Bài 1

a/ \(5.\frac{7}{5}=7\)

b/ \(\frac{3}{4}.\frac{7}{9}+\frac{1}{4}.\frac{7}{9}=\frac{7}{9}.\left(\frac{3}{4}+\frac{1}{4}\right)=\frac{7}{9}.1=\frac{7}{9}\)

c/ \(\frac{1}{7}.\frac{5}{9}+\frac{5}{9}.\frac{1}{7}+\frac{5}{9}.\frac{3}{7}=\frac{5}{9}.\left(\frac{1}{7}+\frac{1}{7}+\frac{3}{7}\right)=\frac{5}{9}.\frac{5}{7}=\frac{25}{63}\)

d/ \(4.11.\frac{3}{4}.\frac{9}{121}=44.\frac{3}{4}.\frac{9}{121}=33.\frac{9}{121}=\frac{27}{11}\)

Bài 2

a/ \(-\frac{16}{7}\)

b/ \(\frac{10}{3}\)

c/ \(\frac{2}{17}\)

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ta có chỉ tồn tại bộ số

\(\left(20+25+35\right)=2\times40\)

thế nên Số cam lúc đầu là :

20 +25 +35 = 80 quả cam

5 X\(^{4^n}\)+13=1293

5x\(4^n\)=1293-13=1280

\(4^n\)=1280 /5=256=\(4^4\)

n=4

Vậy n=4

\(5\times4^n+13=1293\)

\(5\times4^n=1293-13\)

\(5\times4^n=1280\)

\(4^n=1280:5\)

\(4^n=256\)

\(4^n=4^4\)

\(\Rightarrow n=4\)