Question

3sobre4=9sobre?=?sobre20=30sobre?=?sobre80

Answer 1

$Ol\acute{a},~boa~tarde!\\\\Utilizando~a~regra~dos~meios~pelos~extremos.\\\\\dfrac{3}{4}=\dfrac{9}{?}\\3?=4\cdot9\\3?=36\\?=\dfrac{36}{3}\\?=12\\\\\dfrac{3}{4}=\dfrac{?}{20}\\4?=20\cdot3\\4?=60\\?=\dfrac{60}{4}\\?=15\\\\\dfrac{3}{4}=\dfrac{30}{?}\\3?=30\cdot4\\3?=120\\?=\dfrac{120}{3}\\?=40\\\\\dfrac{3}{4}=\dfrac{?}{80}\\4?=80\cdot3\\4?=240\\?=\dfrac{240}{4}\\?=60\\\\\\\boxed{\boxed{\dfrac{3}{4}=\dfrac{9}{12}=\dfrac{15}{20}=\dfrac{30}{40}=\dfrac{60}{80}}}\\\\\\Espero~ter~ajudado.\\Bons~estudos!~=)$