Simplify: same units, then divide
Divide: total parts, one part = total÷parts
% of amount: × the decimal
Up: ×(100+x)% · Down: ×(100−x)%
Profit = sell − cost
% profit = profit÷cost×100
% error = error÷real×100
F = P(1+i)ₜ
P = F ÷ (1+i)ₜ
i = rate as a decimal
Gross tax = rate × income
Net tax = gross − credits
Take-home = income − net tax
€1=$x: €→$ ×x, $→€ ÷x
Cross-rate: go through the euro