The half-life of a radioactive element is 130 days, but your sample will not be useful to you after 80% of the radioactive nuclei originally present have disintegrated. About how many days can you use the sample? Round to the nearest day.

We can use the sample about 42 days.

Step-by-step explanation:

Decay Equation:

Integrating both sides

When t=0, N= = initial amount

.......(1)

                            .........(2)

Logarithm:

130 days is the half-life of the given radioactive element.

For half life,

,   days.

we plug all values in equation (1)

We need to find the time when the sample remains 80% of its original.

We can use the sample about 42 days.

36 girls

step-by-step explanation:

the 48 boys is 4 parts of the ratio

divide 48 by 4 to obtain one part of the ratio

= 12 ← 1 part of the ratio

3 parts = 3 × 12 = 36 ← number of girls

b) 1/3 * 1/5

step-by-step explanation:

each smallest part is 1/15 since the written one is 1/5 and it's divided by 3 equal parts. (1/5 )/3 = 1/15 = 1/3 * 1/5