Calculate the 3rd quartile, 5th decile, and 90th percentile for the given grouped data.

The 3rd quartile is the 75th percentile ($P_{75}$).
Position $= \frac{3N}{4} = \frac{3 \times 40}{4} = 30$.

The 30th observation falls in the class with a cumulative frequency of 30, which is 60-69 (true boundaries 59.5 - 69.5).
$L = 59.5$
$cf_b = 20$ (cumulative frequency of the class before 60-69)
$f_p = 10$ (frequency of the class 60-69)
$w = 10$

The 3rd quartile is $\boxed{\text{69.5}}$.

The 5th decile is the 50th percentile ($P_{50}$).
Position $= \frac{5N}{10} = \frac{5 \times 40}{10} = 20$.

The 20th observation falls in the class with a cumulative frequency of 20, which is 50-59 (true boundaries 49.5 - 59.5).
$L = 49.5$
$cf_b = 13$ (cumulative frequency of the class before 50-59)
$f_p = 7$ (frequency of the class 50-59)
$w = 10$

The 5th decile is $\boxed{\text{59.5}}$.

Position $= \frac{90N}{100} = \frac{90 \times 40}{100} = 36$.

The 36th observation falls in the class with a cumulative frequency of 36, which is 70-79 (true boundaries 69.5 - 79.5).
$L = 69.5$
$cf_b = 30$ (cumulative frequency of the class before 70-79)
$f_p = 6$ (frequency of the class 70-79)
$w = 10$

The 90th percentile is $\boxed{\text{79.5}}$.