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. select the system equations that represents the problem.
j - 2m = 0
m - 2j = 0
j + m = 21
j = m + 2

Which is a logical conclusion based on the given information? a. figure abcd is a rhombus by the definition of a rhombus. b. segment ac is congruent to segment dc by cpctc. c. angle acb is congruent to angle adc by the angle-side-angle theorem. d. triangle acd is congruent to triangle cab by the hypotenuse-leg theorem.