The effective resistance of many individual resistors in series is the sum of the individual resistances.
50 items in series, each with a resistance of 15 ohms, are equivalent to a single resistance of (15 x 50) = 750 ohms.
By the way . . . If one of the little bulbs in series burns out, all the others do NOT burn out. They simply get turned off and go dark, because the current can't flow through the one that did.
