Answer:

Step-by-step explanation:
Urn U1: 3 red and 2 yellow marbles, in total 5 marbles.
The probability to select red marble is
Urn U2: 3 red and 7 yellow marbles, in total 10 marbles.
The probability to select red marble is
Urn U1: 1 red and 4 yellow marbles, in total 5 marbles.
The probability to select red marble is
The probability to choose each urn is the same and is equal to 
Thus, the probability that the marble is red is

-10 = -10 + 7m
Add 10 to each side: 0 = 7m
Divide each side by 7 : 0 = m
Please take a second and just LOOK at the equation you started with.
It says:
-- You have this number '-10' .
-- Your number -10 is going to be exactly the same size
after you add 7 of something to it.
The ONLY way that could happen is if the 'something' is 'nothing' .
' m ' must be nothing.
8 and 9 because 8 x 8= 64 and 9 x 9 = 81. Between 64 and 81 is 75.
Subtracting a negative number is the same as adding a positive number
assume your number is n
n - (-10) = 6
n + 10 = 6
subtract 10 from both sides