spielend
effortlessly
With great ease and without noticeable effort, almost as if it were a game.
Sie hat die schwierige Aufnahmeprüfung spielend bestanden.
She passed the difficult entrance exam effortlessly.

effortlessly / playing
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effortlessly
With great ease and without noticeable effort, almost as if it were a game.
Sie hat die schwierige Aufnahmeprüfung spielend bestanden.
She passed the difficult entrance exam effortlessly.

The word 'spielend' is the present participle of the verb 'spielen' (to play). This form can either literally mean that someone is currently playing, or figuratively that something is very easy—as easy as a game.
Er löste die komplizierte Matheaufgabe spielend.
He solved the complicated math problem effortlessly.
You can recognize the meaning from the grammar: If 'spielend' is used as an invariable adverb, it means 'with ease'. If it is declined as an adjective before a noun, it describes a person who is currently playing.
Sie hat das Rennen spielend gewonnen.
She won the race effortlessly.
In the meaning 'with ease', 'spielend' is often combined with verbs of succeeding. Typical phrases are 'spielend schaffen' (to manage effortlessly), 'spielend gewinnen' (to win effortlessly), 'spielend meistern' (to master effortlessly), or 'spielend bestehen' (to pass effortlessly).
Er hat die Prüfung spielend geschafft.
He managed the exam effortlessly.
In the literal meaning, 'spielend' is used like a normal adjective before a noun and is declined accordingly. It then describes the subject that is currently playing.
Ein spielendes Kind hat den Ball auf die Straße geworfen.
A playing child threw the ball into the street.
simple / just
'einfach' and 'spielend' can both express that something is easy. 'einfach' is the normal word for something that is not complicated. 'spielend' means that something is done with great ease, almost as if it were a game. It sounds stronger and is often used with verbs like 'schaffen' (to manage), 'lösen' (to solve), or 'gewinnen' (to win). The words can be confused because both express ease. But 'spielend' does not fit every simple thing, and in another context it can also mean that someone is currently playing.

Die Klasse löste die ersten Aufgaben spielend, weil sie gut geübt hatte.
The class solved the first exercises effortlessly because they had practiced well.
Ihr findet den Weg einfach, wenn ihr der Karte folgt.
You will find the way easily if you follow the map.