Question 661726
For all percentage problems, remember that the basic equation or formula is that the percentage is a ratio of some “part” divided by some “whole” or standard amount. That doesn't seem precise, but I think that language is helpful for all percentage problems. By part and whole, I mean that there will always be a number that is to be compared to another. 

The basic formula or equation is always:   Percentage = part/whole

In percentage problems, you are given 2 of the following 3 items:
the percentage
the part
the whole

Write the basic equation, substitute in what you know, and solve for what remains unknown.

In this case, you are given the percentage and the part because there was some larger amount of apples that the 420 is a part of. 402 is the amount of apples LEFT. The 40% is what was SOLD. The two must match. If 40% was sold, then 60% is what is left.

pct=part/whole
60% = 420/x
x*60% = 420/x*x
x*60% = 420*1/x*x
x*60% = 420*1
x*60% = 420
x*60%*1/60% = 420*1/60%
x*1 = 420*1/60%
x = 420/60%
x = 420/60%

x = 700 apples

Remember 60% = .60