Let us keep that there are 100 mangoes becoz of percent.
then if 40% mangoes are sold then out of 100 40 mangoes are sold. then remaining mangoes are 100-40=60. then,
if for 100,60 are remaining then for how much are 420 apples remaining.
100:60:x:420
x=100x420/60
=700.
he had 700 mangoes originally