Which of the following is the smallest ratio?

$\frac{5}{6}, \frac{7}{9}, \frac{11}{12}, \frac{13}{18} $

A hemispherical bowl has a radius of 7 cm. It is to be painted from inside as well as outside. Find the cost of painting it at the rate of Rs.40 per 20 cm²? (Take thickness of the bowl as negligible).

If A, B and C can do a work in 6 days, 9 days and 11 days, respectively, then, working together, how many days will they take to do the work?

Rini has mixed two colours 'C₁' and 'C₂' in the ratio 2 ∶ 3. If the rate of the colour 'C₁' is ₹500 per unit and she is selling the mixture of the two colours at ₹650 per unit at breakeven price, then what is the rate (in ₹) per unit of the second colour, that is, ' C₂'?