Answer:
31 st term
Step-by-step explanation:
The n th term of an AP is calculated as
[tex]a_{n}[/tex] = a₁ + (n - 1)d
where a₁ is the first term and d the common difference
Here a₁ = 3 and d = 15 - 3 = 12
Calculate the 21 st term
a₂₁ = 3 + (20 × 12 ) = 3 + 240 = 243
Add 120 to 243 = 363, thus
3 + 12(n - 1) = 363 ( subtract 3 from both sides )
12(n - 1) = 360 ( divide both sides by 12 )
n - 1 = 30 ( add 1 to both sides )
n = 31
That is the 31 st term is 120 more than the 21 st term
