When substituting yarn that comes in different yardages, you can need to work out the total yardage and see how many balls you need to make it.
Eg. If you needed 4 balls of 80m yarn, your total yardage is 4 x 80 = 320;
So if you were substituting a yarn that came in 50m balls, you would need the total length needed for the project divided by the length of the balls you’re using.
That means you need 320/50 = 6.4 balls, so you’d buy 7 balls of the new yarn.