Question

in a row of boys if a who is 10th from the left and b who is 9 from the right interchange position a becomes 15th from the left how many days are there in a row

Answer 1

a and be interchange so a is 15 from left be is 10 and in middle 4 are there so
we will add 10+4+9=23

Answer 2

Interchange means swap ,SO A will go to B's posn and B will go A's position so after B there are 8 persons are there 
Now A's is position is 15 ,so 15+8=23= total no. of boys