Zoho CRM errors - creating formulas

Zoho CRM errors - creating formulas

I'm having problems creating formulas.
For example: Tostring(495/0.9)
In the form above it returns the correct result.
But when I return a string it does the wrong calculation, it only does the right calculation when I return a decimal.
I am creating the formula as follows:
Tostring((Tonumber(${Produtos.Tempo de Fabricação}))/((Tonumber(${Produtos.% Eficiência})/100)))