1) 3^ (x-5) = (3⁹)⁴ : (3²)¹⁰
3^ (x-5)= 3³⁶ : 3²⁰
3 ^(x-5)= 3¹⁶
x-5=16 => x= 16+5= 21
2) a/b= 6/8
a/6=b/8=k
a=6k
b=8k
a+b=70
6k+8k=70
14k=70
k=5
a=6*5=30
b=8*5=40
3)a/x=b/y=c/z =6
a/x=6=> a=6k ∧ x=k
b/y=6=> b=6k ∧ y=k
c/z=6 => c=6k ∧ z=k
(a+b+c)/(x+y+z)= (6k+6k+6k)/(k+k+k)= 18k/3k= 6
