Emma pays $25/month as installments for a t. v screen. how many months will she take to pay a total of $900 in installments?

$25 ... 1 month (installments for a TV screen)
$900 ... x months

If you would like to know how many months will she take to pay a total of $900, you can calculate this using the following steps:

25 * x = 900 * 1    /25
x = 900 / 25 
x = 36 months    /12
x = 3 years

Result: 3 years equals to 36 months.