Question 1101101
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Bruce has

(x of 5-cent stumps) + (6x 10-cent stamps) + (y 20-cent stumps)


Hence, you have these two equations:

(1)  x  + 6x      +    y =  72           (counting stamps)
(2)  5x + 10*(6x) + 20*y = 840  cents    (counting stamps' values) 


Simplify the equations

(1)   7x +   y =  72,
(2)  65x + 20y = 840.


From (1), express  y = 72-7x  and substitute it into eq(2). You will get

65x + 20*(72-7x) = 840  ====>

65x + 1440 - 140x = 840  ====>

-75x = 840 - 1440 = -600  ====>  x = {{{(-600)/(-75)}}} = 8.


So, Bruce has  8  5-cent stamps;  8*6 = 48  10-cent stamps  and  72 - (8+48) = 16    20-cent stamps.


<U>Answer</U>.  Bruce has  8  5-cent stamps;  48  10-cent stamps  and  16  20-cent stamps.


<U>Check</U>.  5*8 + 10*48 + 20*16 = 840.  ! Correct !
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