Color envelopes are packed by the hundred in cardboard boxes. It takes the stationer’s five
seconds to bring down a box from the shelf. Removal of one envelope from the box takes one
second (no matter whether he pulls the envelopes out one by one or counts them out in the box
and takes out the required quantity). The stationer can deal with envelopes of one color only at
a time and can’t possibly take down boxes with, for example, green and blue envelopes,
simultaneously. A customer has asked for 10 green, 10 blue and 60 yellow envelopes. How long
will the shop owner take (at his fastest) to hand the customer the envelopes he asked for?