Michael has 90% as many marbles as Justin. If Michael gives away 20% of the marbles and Justin gives away 60% of his marbles, they will have 504 marbles altogether. How many marbles do the two of them have?

	Michael	Justin	Total
Comparing Michael and Justin at first	9x5 = 45 u	10x5 = 50 u
Before	5x9 = 45 u	5x10 =  50 u
Change	- 1x9 = - 9 u	- 3x10 = - 30 u
After	4x9 = 36 u	2x10 = 20 u	504

90% = ⁹⁰₁₀₀ = ⁹₁₀

20% = ²⁰₁₀₀ = ¹₅ 

60% =⁶⁰₁₀₀ = ³₅ 

The number of marbles that Michael has at first is repeated.  Make the number of marbles that Michael has the same.  LCM of 9 and 5 is 45 u.

The number of marbles that Justin has at first is repeated. Make the number of marbles that Justin has the same. LCM of 10 and 5 is 50 u.

Number of marbles in the end
= 36 u + 20 u
= 56 u

56 u = 504

1 u = 504 ÷ 56 = 9

Total number of marbles that the two of them have
= 45 u + 50 u
= 95 u
= 95 x 9
= 855

Answer(s): 855