
David W. answered 05/16/16
Tutor
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Experienced Prof
If possible, assign a variable to the value the problem asks you to find. Let's use: T = total number of birds.
Also, let S = number of sparrows
P = number of pigeons
D = number of ducks
Translate:
"all sparrows but six" means S = T - 6
"all pigeons but six" means P = T - 6
"all ducks but six" means D = T - 6
Also,
S + P + D = T (the total number of birds in the bag)
Now, the math:
S = T - 6
P = T - 6
D = T - 6
--------------- (add the three equations)
S+P+D = 3T - 18
T = 3T - 18 (substitute T for S+P+D)
T + 18 = 3T (add 18 to both sides)
18 = 2T (subtract T from both sides)
9 = T [There were nine birds in the bag]
Check:
3 sparrows (this is all but 6)
3 pigeons (this is all but 6)
3 ducks (this is all but 6)
Total 9 birds in the bag.