lower triangular matrix [15][15] have first row 1 element, 2nd row 2 element, 3rd row 3 element......15th row 15 element.
Now for [10][6], we need to cross 9 row and when we reach in 10th row we have to cross 5 element. than we will reach beginning of [10][6].
then number of element passed are (1+2+3...+9)+5=50
then the address= 1000+50=1050
Note: array's first element should have the address 1000. mean either [1][1] or [0][0] you are assuming as array's first element should have the address 1000. on the basis of that you will get only 1050 as answer address for [10][6] what ever you assume earlier