Given that:
Time to fill the bathtub with both the cold and hot water faucets = 7 minutes
Time to fill the bathtub only with the hot water faucet = 17 minutes
Find the time to fill the bathtub only with the cold water faucet.
Let the cold water faucet will take x minutes to fill the tab. Then

Solve for x.

Cold water faucet will take 11.9 minutes to fill the tub.