The average of the first three numbers out of four numbers is 18 and that of the last three numbers is 20. If the last number is 25, what is the first number?
- A28
- B36
- C35
- D19
Solution & Step-by-step Explanation
Let the four numbers be a,b,c, and d.
Given:
Average of first three numbers (a,b,c)=18⟹a+b+c=3×18=54
Average of last three numbers (b,c,d)=20⟹b+c+d=3×20=60
Subtracting the first equation from the second equation:
(b+c+d)−(a+b+c)=60−54
d−a=6
We are given that the last number d=25:
25−a=6⟹a=25−6=19
Thus, the first number is 19.
Given:
Average of first three numbers (a,b,c)=18⟹a+b+c=3×18=54
Average of last three numbers (b,c,d)=20⟹b+c+d=3×20=60
Subtracting the first equation from the second equation:
(b+c+d)−(a+b+c)=60−54
d−a=6
We are given that the last number d=25:
25−a=6⟹a=25−6=19
Thus, the first number is 19.