Question 905686
(1) {{{ I + 5M = 225 }}}
(2) {{{ I + 10M = 375 }}}
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Subtract (1) from (2)
(2) {{{ I + 10M = 375 }}}
(1) {{{ -I - 5M = -225 }}}
{{{ 5M = 150 }}}
{{{ M = 30 }}}
and
(1) {{{ I + 5*30 = 225 }}}
(1) {{{ I = 225 - 150 }}}
(1) {{{ I = 75 }}}
The initial fee is $75
The monthly fee is $30 
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check:
(2) {{{ I + 10M = 375 }}}
(2) {{{ 75 + 10*30 = 375 }}}
(2) {{{ 375 = 375 }}}
OK