So first, find out the total amount of money you make in 3 months.
If you make 1200 dollars a month, then in 3 months, you can make 3600 dollars. This is because 1 month salary + 1 month salary + 1 month salary = 3 month salary, and 1 month salary = 1200. So 1200 + 1200 + 1200 = 3600.
Since you're saving 2/5 of that, the simplest method to do this is to divide by 5, to find 1/5 of 3600, then multiply by 2 to find 2/5.
3600/5 = 720
720 * 2 = 1440
So you can save $1440 in three months.