John is twice as old as Mary. The sum of their ages is 21. How old is Mary?
1 answer:
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
*HOPE I HELPED!*
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