How many terms of the AP :24, 21, 18,...........,must be taken so that their sum is 78?

How many terms of the AP :24, 21, 18,...........,must be taken so that

1.	How many terms of the AP :24, 21, 18,...........,must be taken so that their sum is 78?
Answer» See ncert on page 110 Correct answer is 4 or 13 both are correct A=24d=a2-a1d=21-24d=-3an =a+ (n-1)d78=24+(n-1)(+3)78=24+3n-378=24-3+3n78=21+3n78-21=3n75=3n3n=75n=75/3n=25 A=24D=a2-a1D=21-24D=-3An=a+(n-1)d78=24+(n-1)(-3)78=24-3n