In a row of boys, If A who is 10th from the left and B who is 9th from the right interchange their positions, A becomes 15th from the left. How many boys are there in the row?

Rohit is ranked ${18}^{\mathrm{th}}$, from the top in his class. When counted from the bottom, he is ranked ${36}^{\mathrm{th}}.$ What is the total number of students in the class$?$

There are $50$ buildings in a row. The green building is $14\mathrm{th}$from the right end. What is its position from the left end?

A pole is taller than a Giraffe which is taller than a Tree. A signal is shorter than a pole, but taller than a building which is taller than a Giraffe. Who is the shortest ? 

1. Signal
2. Giraffe
3. Tree
4. Building

Five balls $\mathrm{L}1, \mathrm{L}2, \mathrm{L}3, \mathrm{L}4 \mathrm{and} \mathrm{L}5$, are kept one above the other (not necessarily in the same order). $\mathrm{L}1$, is just above $\mathrm{L}5$, and just below $\mathrm{L}4$. $\mathrm{L}2$, is just above $\mathrm{L}3$, and just below $\mathrm{L}5$ . How many balls are above $\mathrm{L}2$?

Ravi is $7$ ranks ahead of Sumit in a class of $39$. If Sumit's rank is seventeenth from the last, what is Ravi's rank from the start?