The fifth term of an arithmetic progression is 19 and the fourteenth term is 15. Find the, first term, common difference, and sum of the first eighty terms.

Answer 1

remember that t(n) = a+(n-1)d

t(8) = a+7d
t(4) = a+3d
it said: the 8th term is twice the 4th term ---> a+7d = 2(a+3d), simplify this

then it said: the 20th term is 40 ---> a+19d = 40

Solve these two equations.