Explicație pas cu pas:
[(20×24-17×25)+ X] ÷2 + (90÷2) ÷5 =421
[(480-425)]+X] ÷2+45÷5 =421
(55+X)÷2+9=421
(55+X)÷2=421-9
(55+X)÷2=412
55+X=412×2
55+X=824
X=824-55
X=819