Question 370502
D=Duck
d=Duckling
(3D + 2d = 77) 
3D=77-2d
D=(77-2d)/3
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Replace D with (77-2d)/3 & solve for d.
(4D + 3d = 107)
4(77-2d)/3+3d=107
(308-8d)/3+3d=107
(308-8d+3*3d)/3=107
(308-8d+9d)/3=107
(308+d)/3=107 cross multiply.
308+d=3*107
308+d=321
d=321-308
d=13 ans.
D=(77-2*13)/3
D=(77-26)/3
D=51/3
D=17 ans.
Proof:
4*17+3*13=107
68+39=107
107=107