10 men, working 6 hours a day can complete a work in 18 days. How many hours a day must 15 men work to complete the work in 12 days ?

Answer: Option C

Explanation:

Although we can first calculate the cost of one toy and then can multiply it with 40 to get the result.
but we will solve it with chain rule so that we can prepare ourself for other complex problems which we can solve easily by applying this rule.

Let the required cost be Rs. x
More toys means More Cost. (Direct Proportion)

\begin{aligned}
15:40 :: 240:x \\
=> x = \frac{240*40}{15} \\
=> x = 640
\end{aligned}

So 40 toys will cost Rs 640

Answer: Option B

Explanation:

Less Cows, More days (Indirect Proportion)
Less bags, Less days (Direct Proportion)

\begin{aligned}
\begin{bmatrix}Cows & 1 & 40 \\Bags & 40 & 1 \end{bmatrix} :: 40 : x \\
=> x * 40 * 1= 40 * 1 * 40 \\
=> x = 40
\end{aligned}

Answer: Option C

Answer: Option C

Explanation:

Please always remember, that what exactly we need to calculate before applying chain formula. As in this question we need to find how many mats, RIGHT !
So we will compare everything with "number of mats"
As,

More men, More number of mats (Direct Proportion)
More days, More number of mats (Direct Proportion)

So, it can be solved as,

\begin{aligned}
\begin{bmatrix}Weavers & 4 & 8 \\Days & 4 & 8 \end{bmatrix} :: 4 : x \\

=> x*4*4 = 8*8*4 \\
=> x = 16
\end{aligned}

Answer: Option C

Explanation:

More Men, Less Hours {Indirect Proportion}
Less Days, More Hours {Indirect Proportion}
\begin{aligned}
\begin{bmatrix}Men & 15 & 10 \\Days & 12 & 18 \end{bmatrix} :: 6 : x \\
=> x * 15 * 12 = 10 * 18 * 6 \\
=> x = \frac{10*18*6}{15*12} \\
=> x = 6
\end{aligned}