A packet contains number cards. Sally picked cards with numbers 2, 4, 5 and 9 from the packet. Which of the following sets has prime numbers only?

A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself.

• 2 is a prime number.
• 4 is not a prime number ($4 = 2 \times 2$).
• 5 is a prime number.
• 9 is not a prime number ($9 = 3 \times 3$).

• A. 2 and 4: Contains 4, which is not prime.
• B. 2 and 5: Both 2 and 5 are prime numbers.
• C. 4 and 9: Contains 4 and 9, which are not prime.
• D. 5 and 9: Contains 9, which is not prime.

The correct option is B.

2. Largest number of groups

Number of boys = 120
Number of girls = 180
Number of teachers = 24

$120 = 2 \times 60 = 2 \times 2 \times 30 = 2 \times 2 \times 2 \times 15 = 2^3 \times 3 \times 5$
$180 = 2 \times 90 = 2 \times 2 \times 45 = 2 \times 2 \times 3 \times 15 = 2^2 \times 3^2 \times 5$
$24 = 2 \times 12 = 2 \times 2 \times 6 = 2 \times 2 \times 2 \times 3 = 2^3 \times 3$

Common prime factors are 2 and 3.
The lowest power of 2 is $2^2$ (from 180).
The lowest power of 3 is $3^1$ (from 120 and 24).

The largest number of groups formed is 12.

The correct option is A.

3. Temperature change

Initial temperature = $-8^\circ C$.

Add this increase to the initial temperature.
Final temperature = $-8^\circ C + 15^\circ C$
Final temperature = $7^\circ C$

The correct option is C.

4. Extra time for planting seedlings

Allocated time per seedling = $\frac{1}{5}$ of an hour.
Actual time taken per seedling = $\frac{1}{4}$ of an hour.

Extra time per seedling = Actual time - Allocated time

Total extra time = (Extra time per seedling) $\times$ (Number of seedlings)

Option D matches this calculation.

The correct option is D.

3 zimekamilika, zimebaki 2 leo. Unapiga hatua.