What is the average weight of students at a school that has 40% boys and 60% girls? Every single boy weighs 200 lbs and every single girl weighs 100 lbs.

Assume there are 100 students at the school.

The 40 boys weigh 40 x 200 lb = 8000 lb

The 60 girls weigh 60 x 100 lb = 6000 lb

The total weight of the students divided by the number of students gives the average:

$$\frac{14 000\,lb}{100}= 140 \,lb$$

There are 2 kinds of pears ; pear-160 with a mass of 160 g and an abundance of 30 %, pear-120 with a mass of 120 g and an abundance of 70 %. What is the average mass of pears?

Assume there are 100 pears in a sample.

Then 30 are pear-160 with a total mass of 30 x 160 g = 4800 g

And 70 are pear-120 with a total mass of 70 x 120 g = 8400 g

The total weight of the pears divided by the number of atoms gives the average:

$$\frac{13200\,g}{100}= 132\,g$$