Let the third number be x. Then second is 2x, first is 4x. Sum = x + 2x + 4x = 7x. Average = 7x/3 = 77. So, 7x = 231, x = 33. The first number is 4x = 4 * 33 = 132.
7514
Average marks of 10 students is 60. If the marks of one student is accidentally read as 30 instead of 80, what is the correct average?
Difference = 80 - 30 = +50. This means the sum is short by 50. Adding 50 to the sum increases the average by 50 / 10 = 5. Correct average = 60 + 5 = 65.
7515
The average of 8 numbers is 20. The average of the first two numbers is 15.5 and the average of the next three is 21 1/3. If the 6th number is 4 less than the 7th and 7 less than the 8th, what is the 8th number?
Total sum = 160. Sum of first 2 = 31. Sum of next 3 = 3*(64/3) = 64. Sum of remaining 3 = 160 - 31 - 64 = 65. Let 6th be y, 7th be y+4, 8th be y+7. y + y+4 + y+7 = 65 => 3y + 11 = 65 => 3y = 54 => y = 18. The 8th number is y+7 = 18+7 = 25.
7516
If the average of 5 observations x, x+2, x+4, x+6, x+8 is 11, then what is the average of the last three observations?
The average of this arithmetic sequence is the middle term, x+4. So x+4 = 11, which gives x=7. The last three observations are 11, 13, 15. Their average is the middle one, 13.
7517
The average temperature from Monday to Thursday is 38°C and from Tuesday to Friday is 40°C. If the temperature on Monday was 30°C, what was the temperature on Friday?
