The average number of pages Sarah read in a certain number of days was 18. In the next four days, Sarah read another 99 pages. As a result, the average number of pages she read increased to 21. How many days did Sarah read in total?
| |
At first |
In the end |
| Number |
1 u |
1 u + 4 |
| Value |
18 |
21 |
| Number x value |
18 u |
21 u + 84 |
Number of days that Sarah read at first = 1 u
Number of pages that Sarah read at first
= 1 u x 18
= 18 u
Number of pages that Sarah read in the end = 18 u + 99
Number of days that Sarah read in the end = 1 u + 4
Number of pages that Sarah read in the end
= 21 x (1 u + 4)
= 21 u + 84
21 u + 84 = 18 u + 99
21 u - 18 u = 99 - 84
3 u = 15
1 u = 15 ÷ 3 = 5
Number of days that Sarah read in the end
= 1 u + 4
= 5 + 4
= 9
Answer(s): 9
