Question 919186
Let {{{ d }}} = number of dimes he has
Let {{{ q }}} = number of quarters he has
----------------------------------
(1) {{{ d = 3.5q }}}
(2) {{{ 10d + 25q = 1560 }}} ( in cents )
-----------------------------------
By substitution:
(2) {{{ 10*3.5q + 25q = 1560 }}}
(2) {{{ 35q + 25q = 1560 }}}
(2) {{{ 60q = 1560 }}}
(2) {{{ q = 26 }}}
---------------
and
(1) {{{ d = 3.5q }}}
(1) {{{ d = 3.5*26 }}}
(1) {{{ d = 91 }}}
----------------
There are 91 dimes
check:
(2) {{{ 10d + 25q = 1560 }}} 
(2) {{{ 10*91 + 25*26 = 1560 }}} 
(2) {{{ 910 + 650 = 1560 }}}
(2) {{{ 1560 = 1560 }}}
OK