First we need to know price per lb. we can calculate that with the info provided. So 10.5/2.5=4.2 So $4.20 per lb. Double check that work before calculating the rest of the problem: 2.5(4.2)=10.5 So if 1 lb cost $4.20 we can multiply 5 1/4 or in decimal form 5.25(4.2)=22.05 So the total cost for 5 1/4 lbs is $22.05
Jack's method is equivalent because of the "law of commutative property", which means that multiplication problems have the same result no matter the order.