The average number of pages Shiyun read in a certain number of days was 12. In the next three days, Shiyun read another 90 pages. As a result, the average number of pages she read increased to 21. How many days did Shiyun read in total?
| |
At first |
In the end |
| Number |
1 u |
1 u + 3 |
| Value |
12 |
21 |
| Number x value |
12 u |
21 u + 63 |
Number of days that Shiyun read at first = 1 u
Number of pages that Shiyun read at first
= 1 u x 12
= 12 u
Number of pages that Shiyun read in the end = 12 u + 90
Number of days that Shiyun read in the end = 1 u + 3
Number of pages that Shiyun read in the end
= 21 x (1 u + 3)
= 21 u + 63
21 u + 63 = 12 u + 90
21 u - 12 u = 90 - 63
9 u = 27
1 u = 27 ÷ 9 = 3
Number of days that Shiyun read in the end
= 1 u + 3
= 3 + 3
= 6
Answer(s): 6
