###Human: After Betty gave Stuart 40% of her marble collection, the number of marbles in Stuart's collection increased to 80. If Betty had 60 marbles, how many marbles did Stuart have initially?, ### Assistant: If Betty had 60 marbles, she gave Stuart 40/100*60=24 marbles.
Since the number of marbles in Stuart's collection increased to 80 after Betty gave him 24 marbles, the number of marbles he initially had is 80-24=56 
 So the answer is :  56