Every iOS rollout from Apple introduces new features we eventually can't live without, but they almost always break something, too. In the case of iOS 11, it's making it difficult to perform TOXIC, a trick that uses the calculator on a borrowed phone. The new operating system changes how the calculator is displayed, which ruins the trick. Fortunately, a discussion over on the Magic Cafe forums has provided a solid workaround.
If you only need to do basic multiplication you can get away with only +0x0 and no bracket in the event the screen won't rotate.
As you enter in each value and press "x" the total will show "0" but the force number will show at the end when you press "=".
A good workaround if you are already in the middle of a routine and can't start over.
For a more full featured toxic app using your phone have a look at iThump with TOXIC+.
iThump is a hefty $29.99, but it would appear to solve the iOS issue quite tidily.