Question 246748
L=largest
m=middle
s=smallest
L=10+s
m=(L+s)/2
L+m+s=42
first substitute (L+s)/2 for m
L+((L+s)/2)+ s=42
now substitute 10+s for L
(10+s)+((10+s)+s)/2+s=42

simplify
10+2s+(10+2s)/2=42
2s+(10+2s)/2=32
multiply by 2
4s+10+2s=64
6s=54
s=9
L=10+s
m=(L+s)/2
substitute 9 in for s