What is the 87th odd positive number?

This an Arithmetic Progression question: (odd number: 1,3,5,7, ...)

1st term a₁ = 1

the common difference is d=2

a term of rank n in a AP = a₁ + (n-1) d, where n = rank of the term.

Now let's apply the formula

a₈₇ = a₁ + (87-1).2

a₈₇ = 173