A messaging app's user count doubled during its first year and then grew by a further 30% during its second year. By what per cent did the user count grow over the two years taken together?

Take the starting count as 100. After doubling it is 200, and after a further 30% it is $200\times 1.30=260$. Growth is the RISE over the start: 160/100 = 160%. 260% is the finishing count as a per cent of the start - a per cent OF, not a per cent growth, and the trap here. 130% quotes only the second year's factor. 230% adds a doubling read as '200%' to the 30%, which would only be right if both acted on the same base.