a) $400-x÷7+78=200$
$⇒$$400-x÷7=200-78$
$⇒$$400-x÷7=122$
$⇒$$x÷7=400-122$
$⇒$$x÷7=278$
$⇒$$x=278×7$
$⇒$$x=1946$
Vậy $x=1946$
b)$\dfrac{4}{5}÷x+\dfrac{3}{2}÷x=\dfrac{1}{5}$
$⇒$$\dfrac{4}{5}×\dfrac{1}{x}+\dfrac{3}{2}×\dfrac{1}{x}=\dfrac{1}{5}$
$⇒$$(\dfrac{4}{5}+\dfrac{3}{2})×\dfrac{1}{x}=\dfrac{1}{5}$
$⇒$$\dfrac{23}{10}×\dfrac{1}{x}=\dfrac{1}{5}$
$⇒$$\dfrac{1}{x}=\dfrac{1}{5}÷\dfrac{23}{10}$
$⇒$$\dfrac{2}{2x}=\dfrac{2}{23}$
$⇒$$2x=23$
$⇒$$x=\dfrac{23}{2}$
Vậy $x=\dfrac{23}{2}$
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