There are seven numbers in a row. Their average number is 30. The average number of the first three numbers is 28, and the average number of the last five numbers is 33. Find the third number.
28x3 33x5-28x7
=84 165-196
=53
Answer: I just made a mistake
28x3 33x5-30x7
=84 165-210
=39
28x39 33x5-28x7?
=84 165-196
=53
That is to say, the total number of these seven numbers is 7x (multiplied by) 30=210
The total number of the first three numbers is 84 (28x3)
The last five numbers are 165 (33x5)
0 ? 0 * 0 ? 0 ? 0 ? 0 0 means that the third number is both of the two totals (* is the third number)
So only 165-84 is needed
30*7=210
28*3=84
33*5=165
84 165=249
249-210=39
39 ? ? 7*30=210 210-3*28=210-84=126 33*5-126=165-126=39
28*3 33*5-30*7=39
Let the third number be m, and the formula is as follows:
30x7=33x5-m 28x3
According to the formula:
m=33x5 28x3-30x7
m=165 84-210=39
The third number is 39
There are seven numbers in a row. Their average number is 30. The average number of the first three numbers is 28, and the average number of the last five numbers is 33. Find the third number.
28x3 33x5-30x7
=84 165-210
=39
The third one is 39
