My husband uses the same method that I was taught in school in the 1950s - equal addition. He asked the little girl to use that method on this question 8157 - 379. It took her 25 seconds to produce the correct answer:
She then attempted the same question using the school's method. She took 7 minutes to arrive at an answer, which turned out to be incorrect.
Here is the recommended method; it goes through a number of stages, each with the potential to mystify and confuse:
The little girl could not do the addition in her head so she had to write out the numbers
The question hanging around chez Random Distractions all weekend is WHY?