Suppose when searching for a large prime, our first step is to sieve by eliminating multiples of the first n primes. How large should n be to speed up our search by a factor of $10?$ Eliminating all even numbers speeds up our program by a factor of $2,$ since we've eliminated half of the choices that are composite. If we also immediately disqualify multiples of $3,$ then we speed up our program by a total of a factor of $3,$ since we have eliminated $2/3$ of all composites. Check that that argument is correct. Now see what happens when you eliminate all of $2,3$ and $5.$ Again for $2,3,5,7.$ Now try to figure out the mathematical result and use it to solve for n$,$ again solve for n$,$ that is the main goal, has to be a numerical value only