Final answer:
The third number is 70, as the sum of all three numbers equals the average multiplied by the number of numbers. Subtracting the two known numbers from this sum gives the third number.
Step-by-step explanation:
To find the third number when the average of three numbers is 60, and the first two numbers are known to be 50 and 60, we use the formula for the arithmetic mean:
Average = (Sum of all numbers) / (Number of numbers)
Accordingly, the sum of all three numbers should equal the average times the number of numbers. In this case, since the average is given as 60 and there are three numbers, the sum should be:
60 × 3 = 180
Now, we subtract the sum of the two known numbers from this total to get the third number:
180 - (50 + 60) = 180 - 110 = 70
So, the third number is 70.