Question 159655
A plum and a pineapple cost the same as three peaches.
pl + pi = 3pe
or
pl = 3pe - pi
:
 Two plums cost the same as a peach.
2pl = pe

 
How many plums cost the same as a pineapple?
;
pl = 3pe - pi: from the 1st equation
:
Substitute 2pl for pe, from the 2nd equation
pl = 3(2pl) - pi
:
pl = 6pl - pi
:
pl - 6pl = -pi
:
-5pl = -pi
:
5pl = pi (5 plums = 1 pineapple}