Question

which term of an ap 3,8,13,18,.....is78

Answer 1

78=a+(n-1)d
78=3+(n-1)5
75=(n-1)5
15=n-1
n=16

Required answer is 16.

Answer 2

a=3. d=8-3=5 t^n=78. t^n=a+(n-1)d. 78=3+(n-1)5. 78-3=(n-1)5. 75/5=n-1. 15+1=n. n=16