There is 3/4 of a box of cereal remaining. you eat 2/5 of the remaining cereal. what fraction of the box do you eat?

You'll need to multiply (3/4) by (2/5).  That's        3(2)          3(1)            3 
                                                                            = =
                                                                              4(5)          2(5)           10

You eat 3/10 of the box's original contents

Alternatively:  (3/4)(2/5) = (3/2)(1/5) = 3/10.