The simple interest $I$ earned on a principal amount $P$ at an annual interest rate $r$ over $t$ years is given by the formula $I=Prt$. An investment account starts with a principal of $P=1000+20n$ dollars, where $n$ is the number of additional $20 increments added to the initial $1000. The account earns simple interest at an annual rate of 4% for 3 years. If the total interest earned must be at least $150, what is the minimum integer value of $n$?