From the year of 1760 to 1820 or 1840 (one those could be your answer so.
1820years - 1760years =
60years
or if its the other year (1840)
Then try going with 1840years - 1760years = 80 years
So, either 60 years or 80 years would most likely be your answer! (If that's one of your options )