seats are arranged 15 rows in an auditorium. each row has 10 seats more than the back the last has 50 seats find out the total number of seats

Answer 1

This makes no sense.

If the 15th row has 50 seats,
the 14th row has 40 seats
the 13th row has 30 seats
...
the 10th row has 0 seats
the 9th row has -10 seats ????????

check your question

Answer 2

Btw, there are many "Similar Questions" listed below this, check them out,
they match your problem once you fixed it.

Answer 3

If you meant 50 rows, then since
a = 15
d=10
then
S50 = 50/2 (2*15 + 49*10) = ____ seats total