J.R. S. answered • 03/31/17

Tutor

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Ph.D. in Biochemistry--University Professor--Chemistry Tutor

Let M = # coins Mary has

Let J = # coins that John has

Together (M + J) they have 150 coins

Mary has 4x the # that John has. This can be written as M = 4J

So, now you have 2 equations with 2 unknowns

Eq 1. M + J = 150

Eq 2. M = 4J

Substituting Eq 2 into Eq 1 you get 4J + J = 150

5J = 150

J = 30 coins

Then since M + J = 150 M must equal 150 - 30 = 120

So, Mary has 120 coins and John has 30 coins.