Question 988650
Let {{{ a }}} = number of books you have read
Let {{{ b }}} = number of books your friend has read
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given:
(1) {{{ a = 3b }}}
(2) {{{ b = a - 4 }}}
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Substitute (1) into (2)
(2) {{{ b = 3b - 4 }}}
(2) {{{ 2b = 4 }}}
(2) {{{ b = 2 }}}
and
(1) {{{ a = 3*2 }}}
(1) {{{ a = 6 }}}
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You have read 6 books
Your friend has read 2 books
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check:
You've read 3 times as many books as your friend 
OK
Your friend but they've only read 4 less books than you
OK