Final answer:
The sum of two numbers c and d, which are rounded to the nearest 10 as 640 and 420 respectively, has a lower bound of 1050 and an upper bound of 1069.8.
Step-by-step explanation:
To find the lower and upper bounds when a number c is rounded to the nearest 10 and is 640, and another number d is rounded to the nearest 10 and is 420, we first determine the range within which these numbers could lie. The numbers c and d could have been rounded up or down, which means that c could be as low as 635 (if it were 634.9, it would have rounded down to 630) and as high as 644.9 (if it were 645, it would round up to 650). Similarly, d could be as low as 415 and as high as 424.9.
Therefore, the lower bound for c + d is when both numbers are at their minimum: 635 + 415 = 1050. The upper bound is when both numbers are at their maximum: 644.9 + 424.9 = 1069.8. Hence, the sum of c + d has a lower bound of 1050 and an upper bound of 1069.8.