If $20.00 is for 12.25kg of peas ...what weight of peas can i get for $4.00​

2.45kg

First you calculate the price per kg as follows:

If it’s $20 for 12.25 kg, the price per kg is $20/12.25=$1.63 per kg

If you have $4, you can figure out what weight you can get by taking $4/the price per kg, so $4/$1.63, which is 2.45 kg.