in 5 days she made 80 sandcastles.

Question

each day she made 4 fewer castles than the day before.
how many castles did she make each day?

Lisa went on making 4 fewer castles each day.
how many castles did she make altogether?

Answer 1

Let X be the number made the first day.
X + X-4 + X-8 + X-12 + X-16 = 80
5X - 40 = 80
5X = 120
X = 24 is the number made the first day. The numbers made on following days are 20, 16, 12, 8, 4 and 0. Add them up for part 2.

Answer 2

If she starts with N castles
After day 2 N-4
After day 3 N-8
After day 4 N-12
After day 5 N-16

Total is N+(N-4)+(N-8)+(N-12)+(N-16) = 80

solve for N

I got N=24
but check this.

Answer 3

24

Answer 4

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Answer 5

give answer in video with explaination

Answer 6

same