Knitting is all about maths.

Sock yarn comes in hanks of 100 grams. One pair of standard socks for myself or Adrian weighs 51 grams. (His are 1.5 cm shorter in the foot than mine but higher in the calf.) So one hank is just barely not enough for two pairs of socks, which is a bit unfortunate, since both Adrian and I loved this yarn. But I can make us a pair each if I use a different yarn for the sock heel for one of the pairs, which would actually be pretty practical anyway, because it will help us tell our socks apart.

This is like an arithmetics problem for elementary school.

Mum has 100 grams of colourful yarn. She knits one pair of socks using 51 grams of the yarn. After knitting a third sock, using plain brown yarn for the heel, she has 27 grams of the colourful yarn left. How much brown yarn did she use for the sock heel? After finishing the second pair of socks, how much colourful yarn will she have left over for darning?