Question

Average of first three numbers out of four
numbers is 20 and the average of last three
numbers is 51. If the last number is 25, then
find the first number.
25​

Answer 1

Answer:

a+b+c÷3=20

a+b+c=60

b+c+25÷3=51

b+c+25=153

b+c=128

a+128=60

a=-68

Answer 2

Answer:

Sum of 6 numbers =60×30=180

Sum of first 4 numbers =4×25=100

Sum of last 3 numbers =3×35=105

∴ Fourth number =(100+105)−180

=205−180

=25