Set it up as a proportion problem. $2.10 is to x pounds as $3.50 is to 1 pound
=$2.10/x pounds= $3.50/1 pound
Cross multiply
=($2.10 * 1)=($3.50 * x)
=2.10=3.50x
Divide both sides by 3.50
0.6= x
x= 0.6=6/10= 3/5
He can purchase 0.6 (or 3/5 in fraction terms) pounds for $2.10.
Hope this helps! :)
