Question 1012773
Let {{{ b }}} = number of birds at shelter on friday
Let {{{ c }}} = number of cats at shelter on friday
-------------------
(1) {{{ b + c = 24 }}}
(2) {{{ 1.5b + 7c = 74.5 }}}
---------------------
(2) {{{ 15b + 70c = 745 }}}
(2) {{{ 3b + 14c = 149 }}}
Multiply both sides of (1) by {{{ 3 }}} 
and subtract (1) from (2)
-------------------------
(2) {{{ 3b + 14c = 149 }}}
(1) {{{ -3b - 3c = -72 }}}
----------------------
{{{ 11c = 77 }}}
{{{ c = 7 }}}
and
(1) {{{ b + c = 24 }}}
(1) {{{ b + 7 = 24 }}}
(1) {{{ b = 17 }}}
There were 17 birds at the shelter on friday
check:
(2) {{{ 1.5b + 7c = 74.5 }}}
(2) {{{ 1.5*17 + 7*7 = 74.5 }}}
(2) {{{ 25.5 + 49 = 74.5 }}}
(2) {{{ 74.5 = 74.5 }}}
OK