how many pounds of extra lean hamburger that is 7% fat must be mixed with 30 pounds of hamburger that is 15% fat to obtain a mixture that is 10% fat?

Let x be the number of pounds of  extra lean hamburger.

0.07x+30*0.15=(x+30)*0.1

0.07x+4.5 =0.1x+3.

0.1x-0.07x=4.5-3.

0.03x=1.5

x=1.5/0.03

=50 pouns.