Question 831021
Let {{{ a }}} = number of regular keyboards sold
Let {{{ b }}} =  number of wireless keyboards sold
----------------------
(1) {{{ a = 3b }}}
(2) {{{ 84a + 105b = 4998 }}}
-------------------------
Substitute (1) into (2)
(2) {{{ 84*(3b) + 105b = 4998 }}}
(2) {{{ 252b + 105b = 4998 }}}
(2) {{{ 357b = 4998 }}}
(2) {{{ b = 14 }}}
and, since
(1) {{{ a = 3b }}}
(1) {{{ a = 42 }}}
42 regular keyboards were sold
14 wireless keyboards were sold
check:
(2) {{{ 84a + 105b = 4998 }}}
(2) {{{ 84*42 + 105*14 = 4998 }}}
(2) {{{ 3528 + 1470 = 4998 }}}
(2) {{{ 4998 = 4998 }}}
OK