let the number of apples a fruit seller initially has= x;
Since according to the question after selling 40 percent of apples he is left with 420 apples therefore:
x-(40/100)*x=420; eqn 1
Now by solving eqn 1 we get the value of x equal to 700 which is the total amount of apples he initially has therefore the total number of apples he originally had is equal to 700.