The second one is very easy.

Add 10 erasers, the mass increases.

By how much?

306g - 226g = 80g

This tells you the 10 erasers weigh 80g

1 eraser's mass

= 80g ÷ 10
= 8g

25 erasers' mass

= 25 × 8g
= 200g

Mass of box = 226g - 200g = 26g

OR

35 erasers' mass

= 35 × 8g
= 280g

Mass of box = 306g - 280g = 26g


(Your misconception was : you thought mass of box + 10 erasers = 80g. But if you read carefully, the mass increased after adding 10 erasers. There is only 1 box, no other boxes were added)

First question :

Average mass of the 3 children = 38kg

So total mass of the 3 children must be:
38kg × 3 = 114kg

The average mass increased to 40kg when the other two children joined. There are a total of 5 children now.

So now, average mass of 5 children = 40kg
Total mass of 5 children = 40kg × 5 = 200kg

Total mass of Denise and Ethan

= 200kg - 114kg
= 86kg

Alternative method to do first question :

When Denise and Ethan joined the group, the average mass increased to 40kg.

40kg - 38kg = 2kg

This is as good as saying the first 3 children increased by 2kg each.

2kg × 3 = 6kg

We can think of it as Denise and Ethan 'donating' this 6kg to them, to average out or even out the mass.

Since the average is 40kg now, then Denise and Ethan are 'left with' 40kg each.

40kg × 2 = 80kg

Add back the 6kg that was 'donated',

80kg + 6kg = 86kg

80kg +