Question

Given arithmetic rows with U3 = 4 and U27 = -44
The fifteenth tribe is?

Answer 1

U3 = 4

A + (3-1) b = 4
A + 2b = 4 .............. [1]

U27 = -44
A + (27-1) b = -44
A + 26b = -44 .......... [2]

Elimination of press [1] and [2]:
A + 2b = 4
A + 26b = -44
___________--
     -24b = 48
          B = -2

Substitution of the value of b in one equation:
A + 2b = 4
A + 2 (-2) = 4
4 = a + 2 (-2)
4 = a-4
A = 4 + 4
A = 8

U15 = a + (15-1) b
U15 = 8 + 14. (- 2)
U15 = 8-28
U15 = -20