Tolong bantu pakai cara terima kasih

a.
[tex] \frac{ab}{bc} = \frac{aob}{ boc} [/tex]
[tex] \frac{20}{bc}= \frac{40}{60} [/tex]
bc=30cm
b.
[tex] \frac{120}{aob} =\frac{60}{40}
aob=80