Answer:
21
Step-by-step explanation:
Let x, y, z, t, u represent the five numbers. Since their average is 18,
(x + y + z + t + u) / 5 = 18
→ x + y + z + t + u = 90 (1)
If the first number is increased by 1, the second number by 2, the third number by 3, the fourth number by 4, and the fifth number by 5, then the total of the numbers will be
(x+1) + (y+2) + (z+3) + (t+4) + (u+5)
→ x + y + z + t + u + 15 (Substitute x + y + z + t + u from equation 1)
→ 90 + 15 = 105
If the total of increased numbers is 105, then the average is
105 / 5 = 21
