Integers are listed from 700 to 1000. In how many integers is the sum of the digits 10 ?

There are three points P, Q and R on a straight line such that PQ : QR = 3 : 5. If n is the number of possible values of PQ : PR, then what is n equal to ? 

On a chess board, in how many different ways can 6 consecutive squares be chosen on the diagonals along a straight path ?

In the series _b_a_ba_b_abab_aab;

fill in the six blanks ( _ ) using one of the following given four choices such that the series follows a specific order.