2/5 of bulbs were not shining. It is equal to 4 bulbs.
Explanation:
10/10 bulbs equals all 10 bulbs. 3/5 were shining, that is equal to 6/10 of all ten bulbs. (or 60%). 60% of ten is 6 bulbs shining.The number of bulbs that werent shining is 10 - 6=4.