Green’s Dictionary of Slang

(US campus) relatively easy.

1900		E.H. Babbitt ‘College Words and Phrases’ in DN II:i 61: snap, adj. quite easy.
1930		J. O’Hara ‘The Man Who Had to Talk to Somebody’ in New Yorker 11 Oct. 60/3: Williams and I used to work in the same office. It was a snap job for both of us and the pay was poor, so we sat around a lot.
1967		P. Thomas Down These Mean Streets (1970) 70: Living in number 109 was a snap breeze.