Why my iphone 6 is unable to get correct answer for 77-38.5? it said the trsult is 0. what's wrong?

you are tapping the equal sign twice. The first time you get the correct result of 38.5. The second time you erase the result and set the calculator back to 0.

77-38.5 = 38.5. Just hit the = sign once.

If you tap the " = " twice, the calculator will subtract the 38,5 again and the correct result is " 0 ".

Just tried that on my 5s in landscape mode and normal view.

First time I tried this I thought the equal sign did lag a bit, and I had the same phenomenon, but after switching to landscape mode it worked as expected.