You are given an arithmetic progression (an), the difference of which is −4.9, a1 = −6.4. Find a15.

Let’s remember the formula for finding the n-th term of the arithmetic progression (an):

an = a1 + (n – 1) * d, where a1 is the first term of the progression, n is the ordinal number of the sought term in the progression, d is the difference of the progression.

Let’s calculate the fifteenth term of a given progression:

a15 = -6.4 + (15 – 1) * (-4.9),

a15 = -6.4 + 14 * (-4.9),

a15 = -6.4 – 68.6,

a15 = -75.

Answer: a15 = -75.