Question

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