Ok so let the price for 1 red marble be R, and the price of one green marble be G.
We know Roman paid 70 cents for 4 red and 2 green marbles, so 4R + 2G = 70
We also know Bob paid 95 cents for 2 red and 5 green marbles, so 2R + 5G = 95.
However, if Bob bought double of each kind of marble, he would inevitably pay double as well, so 4R + 10G = 190
So if you subtracted the equations,
(4R+10G) - (4R+2G) = 190 - 70
So 8G is 120
G = 15
So a green marble is 15 cents.
Sub this back in:
4R + (2x15) = 70
4R = 40
R = 10
So a red marble is 10 cents.
So of Roger buys one of each, he pays 10 + 15 = 25 cents
Hope this helped
