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John is twice as old as Mary. The sum of their ages is 21. How old is Mary? Let J = John's age and M = Mary's age.
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4 votes
John is twice as old as Mary. The sum of their ages is 21. How old is Mary?

Let J = John's age and M = Mary's age.

User MistahX
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1 Answer

4 votes
j=john
m=mary
j is 2 times as old as mary
j=2 times m

sum is 21
m+j=21
find mary
subsitute

j=2m therefor subsitute 2m for j
m+2m=21
add like terms
3m=21
divide by 3
m=7

mary=7
User Sergei Krivosheenko
by
8.9k points
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