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?
If Betty had 60 marbles, she gave Stuart 40/100*60=<<60/100*40=24>>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=<<80-24=56>>56
#### 56