(a) We have ⌊<em>x</em>⌋ = 5 if 5 ≤ <em>x</em> < 6, and similarly ⌊<em>x</em>/3⌋ = 5 if
5 ≤ <em>x</em>/3 < 6 ==> 15 ≤ <em>x</em> < 18
(b) ⌊<em>x</em>⌋ = -2 if -2 ≤ <em>x</em> < -1, so ⌊<em>x</em>/3⌋ = -2 if
-2 ≤ <em>x</em>/3 < -1 ==> -6 ≤ <em>x</em> < -3
In general, ⌊<em>x</em>⌋ = <em>n</em> if <em>n</em> ≤ <em>x</em> < <em>n</em> + 1, where <em>n</em> is any integer.
I do not understand what is being asked in (c) and (d), so you'll have to clarify...
The answer is 28
work shown in picture below
8 ft= ___ inches
1 ft= 12 inches
8 ft= 12 x 8 inches
8 ft=96 inches
She has 96 inches, but she needs 100 inches. So she needs 4 more inches of lumber.