Question 831074
Let {{{a}}} = pounds of meat with 18% fat needed
Let {{{b}}} = pounds of meat with 15% fat needed
---------------------------------
(1) {{{ a + b = 87 }}}
(2) {{{ ( .18a + .15b ) / 87 = .16 }}}
-------------------------------
(2) {{{ .18a + .15b = .16*87 }}}
(2) {{{ .18a + .15b = 13.92 }}}  
(2) {{{ 18a + 15b = 1392 }}}
Multiply both sides of (1) by {{{15}}} and
subtract (1) from (2)
(2) {{{ 18a + 15b = 1392 }}}
(1) {{{ -15a - 15b = -1305 }}}
{{{ 3a = 87 }}}
{{{ a = 29 }}}
and, since
(1) {{{ a + b = 87 }}}
(1) {{{ 29 + b = 87 }}}
(1) {{{ b = 58 }}}
29 pounds of meat with 18% fat are needed
58 pounds of meat with 15% fat are needed
check:
(2) {{{ ( .18*29 + .15*58 ) / 87 = .16 }}}
(2) {{{ ( 5.22 + 8.7 ) / 87 = .16 }}}
(2) {{{ 13.92 = .16*87 }}}
(2) {{{ 13.92 = 13.92 }}}
OK