Answer: 221 years or in the year 2192
Step-by-step explanation: The 13-year locusts emerge after every 13 years, that is, after every year that is a multiple of 13. The 17- year locusts emerge after every 17 years, that is, after every year that is a multiple of 17. Therefor, if both 13- and 17- year locusts happened to emerge in 1971, such interference would repeat after every x years, where x is a common multiple of 13 and 17. The next interference would occur after the number of years which is the least common multiple of 13 and 17.
