#1**+1 **

5+4+3 =12 - total number of marbles

5/12 =41.67%

Let the number of extra green marbles =G

[G+5] / [3+4+5 + G] =0.75

Solve for G:

(G + 5)/(G + 12) = 0.75

0.75 = 3/4:

(G + 5)/(G + 12) = 3/4

Cross multiply:

4 (G + 5) = 3 (G + 12)

Expand out terms of the left hand side:

4 G + 20 = 3 (G + 12)

Expand out terms of the right hand side:

4 G + 20 = 3 G + 36

Subtract 3 G + 20 from both sides:

**G = 16 - Extra green marbles will be needed to make 75% of the total**.

Proof: 16 + 5 =21 - Green marbles

3 + 4 + 21 = 28 - marbles in total, so that:

21 / 28 =75% - are green marbles.

Guest Feb 16, 2018

edited by
Guest
Feb 16, 2018

#2**+1 **

¸We have a total of 12 marbles....let x be the number of green ones we need to add

[ Green + Added Green ] / [ Total + Added Green ] = .75 = 3/4

[ 5 + x ] / [ 12 + x ] = 3/4 cross-multiply

4 [ 5 + x ] = 3[ 12 + x ] simplify

20 + 4x = 36 + 3x subtract 3x, 20 from both sides

x = 16 ⇒ 16 green must be added

CPhill
Feb 16, 2018