Seth started exercising by doing 9 planks on the first day. Every day, he would do 2 more planks than the day before. How many planks did he do altogether after 9 days?
| |
Number of
planks |
Number of
planks |
Total number
of planks |
| Day 1 |
9 |
7 + (1 x 2) |
9 |
| Day 2 |
9 + 2 |
7 + (2 x 2) |
9 + 11 |
| Day 3 |
9 + 2 + 2 |
7 + (3 x 2) |
9 + 11 + 13 |
| |
Total number
of planks |
Total number
of planks |
| Day 1 |
9 |
(1 + 3 + ... + 9) - (1 + 3 + ... + 7)
= 52 - 42
= 9 |
| Day 2 |
9 + 11 |
(1 + 3 + ... + 11) - (1 + 3 + ... + 7)
= 62 - 42
= 20 |
| Day 3 |
9 + 11 + 13 |
(1 + 3 + ... + 13) - (1 + 3 + ... + 7)
= 72 - 42
= 33 |
| Day 9 |
9 + 11 + 13 + 15 + ... + 25 |
(1 + 3 + ... + 25) - (1 + 3 + ... + 7)
= 132 - 42
= 153 |
Formula:
Number of planks = 7 + (Day x 2)
Number of planks on Day 9 = 7 + (9 x 2) = 25
To find the range of odd numbers not included
= Total number of planks on Day 1 - 2
= 9 - 2
= 7
- Range of odd numbers from 1 to 7 are not counted.
Formula:
To find the number of odd numbers in a range starting from 1 to the last digit
= (Last digit + 1) ÷ 2
Number of odd numbers from 1 to 7
= (7 + 1) ÷ 2
= 4
Number of odd numbers from 1 to 25
= (25 + 1) ÷ 2
= 13
Sum of odd numbers from 1 to 7
= 4
2= 4 x 4
= 16
Sum of odd numbers from 1 to 25
= 13
2 = 13 x 13
= 169
Sum of odd numbers from 9 to 25
= 169 - 16
= 153
Total number of planks after 9 days = 153
Answer(s): 153
