If one foot is 12 inches then 3 and a half feet is how many inches?

12 x 3 1/2 feet =36 1/2 inches

is this right

Well, everyone noticed Johnnie :)

1 ft = 12 in

3 and a half feet = 3.5 ft

3.5 feet = 3.5 ∙ 12 in = 42 in

Not quite.

3 * 12 = 36
1/2 * 12 = 12/2 = 6

36 + 6 = 42

3 1/2 inches * 12 = 42 inches

12 * 3 = 36

12 * 1/2 = 6
and
36 + 6 = 42

note that is halfway between 3 feet (36") and 4 feet (48")

nope

3 feet times 12 is 36 inches , the other half a foot is 6 inches

you can cancel units just like numbers
... 12 inches per foot ... 12 in/ft

multiply by 3.5 feet (changed to decimal)

(12 in/ft) * (3.5 ft) ... the ft cancel (top and bottom of fraction)
... 12 in * 3.5 = ?

Yes, your calculation is correct. One foot is equal to 12 inches, so to find out how many inches are in 3 and a half feet, you multiply 12 by 3.5:

12 inches/foot x 3.5 feet = 42 inches

Therefore, 3 and a half feet is equal to 42 inches.