In how many ways can we select two books from different subjects from among 7 distinct math books, 5 distinct sciencs books and 4 distinct english books?

We want to select :

1 Science book and 1 Math book     or

1 Science book  and 1 English book  or

1 Math book and 1 English book

ways  to do the first thing  +  ways to do the second thing + ways to do the third thing =

5C1 x 7C1                          +     5C1 * 4C1                         +   7C1 * 4C1         =

35                                        +     20                                     +      28                  =

83 ways