The average mass of some parcels in a box was 20kg at first. When another parcel which weighs 10kg is added into the box, the average mass became 18kg. How many parcels are there in the box now?

Miki Theseira wrote:
The average mass of some parcels in a box was 20kg at first. When another parcel which weighs 10kg is added into the box, the average mass became 18kg. How many parcels are there in the box now?
Assume the number of parcels at first be N.
Total mass of N parcels at first -> Nx20 = 20N kg
Additional parcel -> 10 kg
New total mass of (N+1)parcels -> 20N+10 which was also 18(N+1).

20N+10=18(N+1)
20N+10=18N+18
2N=8
N=4

4+1=5
Number of parcels in the box now = 5

Check
Total mass of 4 parcels = 4x20kg = 80 kg
Total mass of 5 parcels = 80 kg + 10 kg = 90 kg
Average mass of 5 parcels = 90 kg/5 = 18 kg

You can refer to Chapter 4.3 of the +hinkingMath book for a number of examples of this style of question.