15. Seventeen year and thirteen year locusts both emerged in 1971. When will be the

next time they both emerge in the same year?

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.