Question 896579
Let {{{ t }}} = time in hours 
Let {{{ s }}} = speed of the T.N.H in km/hr
Equation for commuter airplane:
(1) {{{ 1350 = ( s + 40 )*t }}}
Equation for television news helicopter:
(2) {{{ 1050 = s*t }}}
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(2) {{{ t = 1050/s }}}
Substitute (2) into (1)
(1) {{{ 1350 = ( s + 40 )*( 1050/s ) }}}
(1) {{{ 1350 = 1050 + 42000/s }}}
(1) {{{ 300 = 42000/s }}}
(1) {{{ s = 42000/300 }}}
(1) {{{ s = 140 }}}
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The speed of the T.N.H is 140 km/hr
check:
(2) {{{ t = 1050/s }}}
(2) {{{ t = 1050/140 }}}
(2) {{{ t = 7.5 }}}
(1) {{{ 1350 = ( s + 40 )*7.5 }}}
(1) {{{ 1350 = 7.5s + 300 }}}
(1) {{{ 7.5s = 1050 }}}
(1) {{{ s = 140 }}}
OK