Identify the range boundaries

We need multiples of 7 strictly between 11 and 90. The first multiple of 7 greater than 11 is $14$ ($7 \times 2$). The last multiple of 7 less than 90 is $84$ ($7 \times 12$).

$F = 14, L = 84$

Apply the arithmetic sequence formula

The number of terms $n$ in an arithmetic sequence is given by the formula $n = \frac{L - F}{d} + 1$, where $d$ is the common difference, which is $7$.

$n = \frac{84 - 14}{7} + 1$

Calculate the result

Subtract the first term from the last term and divide by the common difference, then add 1: $n = \frac{70}{7} + 1 = 10 + 1 = 11$.

$n = 11$