You start with 80 and ADD 10 each week
—-> 80 + 10x

Your brother starts with 275 and SUBTRACTS 15 each week
—> 275 – 15x

Set expressions equal to find out when you have same amount as your brother.

80 +10x = 275 -15x

Add 15x to both sides

80 +25x = 275

Subtract 80 from both sides

25x = 195

Divide by 25

x = 7.8  (Round up to 8)

This means in 8 weeks you will have caught up and if fact have more money than your brother.

Since you save 10 per week, in 8 weeks the Total saved = 10*8 = 80