Answer:
It will take 3 months for their accounts to have the same amount and the amount would be $70.
Explanation:
Christina: Her initial amount is $40 and each month she adds $10, so the equation would be 40+10x.
Marlena: Her initial amount is $25 and she add $15 each month, so the equation would be 25+15x
Plug both of these equations into a graphing calculator (Desmos) and look for both lines intersect.