Question

Sammy wants to calculate the time it takes for a ladybug traveling at a speed of 3 inches (in.) per second (sec.) to crawl a distance of 10 feet (ft.).

Starting with the formula d=rt
, Sammy rearranges the formula to highlight time, t, as the quantity of interest.

t=dr


Sammy substitutes the values for rate and time into the formula t=dr
and simplifies to calculate the time.

t=10 ft.3in.sec.t≈3.3 sec.


Sammy determines it takes the ladybug about 3.3
sec. to travel 10 ft.

What mistake did Sammy make?

(1 point)
Responses

Sammy needed to convert 10 ft. to 120 in. by multiplying by 12 in.1 ft.
before substituting.
Sammy needed to convert 10 ft. to 120 in. by multiplying by Start Fraction 12 inches over 1 foot End Fraction before substituting.

Sammy needed to convert 3 in. per sec. to 36 in. per sec. by multiplying by 12 in.1 ft.
before substituting.
Sammy needed to convert 3 in. per sec. to 36 in. per sec. by multiplying by start fraction 12 inches over 1 foot end fraction before substituting.

Sammy needed to convert 3 in. per sec. to in. per minute (min.) by multiplying by 1 min.60 sec.
before substituting.
Sammy needed to convert 3 in. per sec. to in. per minute (min.) by multiplying by start fraction 1 minute over 60 seconds end fraction before substituting.

Sammy did not isolate t properly. He should have used t=rd
.

Answer 1

Sammy forgot to use compatible units. He must convert 10 ft to inches: 10 ft = 10 × 12 = 120 in. Then

t = d/r = 120 in ÷ 3 in/sec = 40 sec.

So the correct choice: "Sammy needed to convert 10 ft. to 120 in. by multiplying by 12 in/1 ft before substituting."