Question 1051628
There are 3 unknowns:
(1) Number of hrs spent sorting mail = {{{ a }}}
(2) Number of hrs spent selling stamps = {{{ b }}}
(3) Number of hrs spent doing supervisory work = {{{ c }}}
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You need 3 equation in order to find 3 unknowns
given:
(1) {{{ a + b + c = 6.5 }}}
(2) {{{ b = 2a }}}
(3) {{{ b = c + 1 }}}
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(2) {{{ a = b/2 }}}
and
(3) {{{ c = b - 1 }}}
Substitute 92) and (3) into (1)
(1) {{{ b/2 + b + b - 1 = 6.5 }}}
(1) {{{ b/2 + 2b = 7.5 }}}
Multiply both sides by {{{ 2 }}}
(1) {{{ b + 4b = 15 }}}
(1) {{{ 5b = 15 }}}
(1) {{{ b = 3 }}}
and
(2) {{{ a = b/2 }}}
(2) {{{ a = 1.5 }}}
and
(3) {{{ c = b - 1 }}}
(3) {{{ c = 3 - 1 }}}
(3) {{{ c = 2 }}}
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She spent 1.5 hrs sorting mail,
3 hrs selling stamps, and 2 hrs 
doing supervisory work
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check:
(1) {{{ a + b + c = 6.5 }}}
(1) {{{ 1.5 + 3 + 2 = 6.5 }}}
(1) {{{ 6.5 = 6.5 }}}
You can check (2) and (3)