Start at the year 2016 and keep incrementing by one.
What is a leap year?
Years divisible by 4 are leap years, except for those that are also divisible by 100. Years divisible by 400 are leap years regardless.
So you will need a series of if-statements in your loop to decide whether you will print the current year or not. Have a counter variable that you increment every time you print a leap year. Once that variable reaches 20, you can end the loop.
Write the pseudo code first based on the rules above, figure out how you can do this using a for-loop, and then translate to C++ code. Post here if you get stuck.