When you were just starting your school life, didn't even know how to add well, and who were there for you? Your precious siblings, or almost all of them.

Ichiro was out on an errand, and well, Saburo is Saburo, so you only had Jiro as a last resort.

"...Double-digit division?" Jiro agreed to help you, not knowing that your homework is math, making his head unable to cope with so much stuff. "I mean, of course I know how to divide! But why don't we start with addition?" Now, he wanted to leave the more complicated stuff aside.