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If Tommy is twice as old as Billy, then we can use variables to represent their ages like this:
B = Billy's Age
B * 2 = Tommy's Age
Then, if their two ages add up to be 2 years less than 79, we can write the whole question in "math language" as:
B + B*2 = 79 - 2
Now we simplify, since 1B added to 2B's is 3B's, we get:
3B = 77
If 3 B's is 77, then 1 B would be 1/3 of 77, or 77/3.
B (Billy's age) is 25 2/3 or 25 years and 8 mos.
Since Tommy is twice that old, his age is 50 years and 16 months or 51 years and 4 mos.