How do I get the temperature of the water?

Water if flowing from 2 different sinks, one is 30 degree the other is 100 degree, the 30 degree one fills the tank in 3 minuites, while the 100 degree one fills it in 7 minuites.

I know I have to apply the proportions to get the average temperature but I don't know exactly how. The proportion is 100 degree water (3:7) 30 degree water. I tried these ways: (100*0.3 + 30*0.7)/2 , but i'm not sure if that's right.

• Hint: This question probably is better asked in a physics forum, but anyway. Forgetting about the calculation, what does you intuition tell you? Should the final temperature be warmer/colder than 30 resp. 100 degrees of either source? Does that match your answer? Also, if you would solve it for arbitrary flow speeds, you have the limiting cases of only water from either source and you know what the general results should give you. – Ronald Blaak Nov 6 '17 at 19:47
• First try to find in how many minutes you fill the tank with the two sinks. See math.stackexchange.com/questions/2507853/…. – cgiovanardi Nov 7 '17 at 19:08