6 pieces of rope lengths vary from 67 to 84 inches. How long could the total length be?
a)360
b)390
c)480
c)540
I know answer must be 480 but how do I get that

What is the smallest number it could be, if all the ropes were the shortest length?

What is the longest the rope could be if they were all the longest length

402
504
That is what I did but I wasn't sure it was right
Thank you