Answer:
Mark has 24
Don has 79
Step-by-step explanation:
Let x = marbles mark has
don has 3x+7
Together they have 103
mark + don = 103
x+ (3x+7) = 103
Combine like terms
4x+7 = 103
Subtract 7
4x = 96
Divide by 4
x = 96/4
x =24
Mark has 24
Don has 103 -24
Done has 79