Hello!
To begin solving this, we must determine the total amount of weight that needed to be divided. Since three people each carried 7.8kg of equipment (before the fourth person joined), the total amount is:
7.8x3= 23.4
After the fourth person joins the group, the weight is divided equally among the climbers:
23.4/4= 5.85
Each climber carried 5.85 kilograms of equipment.
I hope this helps you! Have a fantastic day!