a) \(\frac{20-a}{24-a}=\frac{2}{3}\)
\(\Rightarrow\left(20-a\right).3=\left(24-a\right).2\)
\(\Rightarrow60-3a=48-2a\)
\(\Rightarrow60-48=\left(-2a\right)+3a\)
\(\Rightarrow12=1a\)
\(\Rightarrow a=12:1\)
\(\Rightarrow a=12\)
Vậy \(a=12.\)
b) \(\frac{20-a}{22-a}=\frac{3}{7}\)
\(\Rightarrow\left(20-a\right).7=\left(22-a\right).3\)
\(\Rightarrow140-7a=66-3a\)
\(\Rightarrow140-66=\left(-3a\right)+7a\)
\(\Rightarrow74=4a\)
\(\Rightarrow a=74:4\)
\(\Rightarrow a=\frac{37}{2}\)
Vậy \(a=\frac{37}{2}.\)
c) \(\frac{30-a}{36-a}=\frac{1}{2}\)
\(\Rightarrow\left(30-a\right).2=\left(36-a\right).1\)
\(\Rightarrow60-2a=36-a\)
\(\Rightarrow60-36=\left(-a\right)+2a\)
\(\Rightarrow24=1a\)
\(\Rightarrow a=24:1\)
\(\Rightarrow a=24\)
Vậy \(a=24.\)
d) \(\frac{48-a}{52-a}=\frac{4}{5}\)
\(\Rightarrow\left(48-a\right).5=\left(52-a\right).4\)
\(\Rightarrow240-5a=208-4a\)
\(\Rightarrow240-208=\left(-4a\right)+5a\)
\(\Rightarrow32=1a\)
\(\Rightarrow a=32:1\)
\(\Rightarrow a=32\)
Vậy \(a=32.\)
Chúc bạn học tốt!