You buy a computer for $1200. the value of the computer decreases by 30% each year. find the value of the computer after 4 years.

The amount of 30% is $1200 *30 /100 = $360, 
y1 =$360 (year 1),  so let's find y2
the amount of 30% is $360*30/100 =$108
let 's find y3, 
the amount of 30% is $108*30/100 =$32.4
let 's find  y4
the amount of 30% is $32.4*30/100 =$9.72

so the value of the computer after 4 years is $9.72