updating the variable inside for loop

Try this. I think you should recalculate the total money. Because number of shops is 21, is correct. Please recalculate total money; it can never be 210000.

public class investmentCalculation {
private int income;
private int shop;
private int firstMoney;
private int totalMoney1;
private int shopsAdded;

public investmentCalculation() {
    income = 10000;
    shop = 3;
    firstMoney = 30000;
    totalMoney1 = 0;
    shopsAdded = 0;
}

// here the method to achieve the goal
public int SetFor() {
    for (int i = 0; i < 7; i++) {
        totalMoney1 += income * shop;
        shopsAdded = totalMoney1 / firstMoney;  //Calculate how many shops can be added with the money you have.
        shop += shopsAdded;                     //Increment the shops by the newly added shops.
        totalMoney1 -= firstMoney * shopsAdded; //Decrement the cost required to form the new shops.
    }
    System.out.println("the total money is: " + totalMoney1);
    return shop;
}

/** this class to test the investmentCalculation */
public static void main(String[] args) {
    investmentCalculation TT = new investmentCalculation();
    int A = TT.SetFor();
    System.out.println("the shops are:   " + A);
}

}`