27)
a/3=b/4=c/5 =k
a=3k , b=4k ,c=5k
(a+b+c) : 3=144
3k+4k+5k= 144*3
12k = 144*3
k=144*3:12 = 12*3=36
a=36*3=108
b=36*4=144
c=36*5=180
30)
a/6=b/8=c/10 = k
a=6k ; b=8k ; c=10k
2a+b+c= 15
12k +8k+10k=15
30k=15
k=15/30
k=1/2
a=6/2=3
b=8/2=4
c=10/2=5
15)
3/a=4/b=5/c =1/k
a=3k
b=4k
c=5k
2c+3b-4a=40
10k+12k-12k=40
10k=40
k=40:10
k=4
a=12
b=16
c=20
