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Find the number of surjective (onto) functions $f$ from $\{1,2,3,\ldots,20\}$ to $\{1,2,3,\ldots,20\}$ such that $f(k)$ is a multiple of 3 whenever $k$ is a multiple of 4.
Multiples of 4 in $\{1,\ldots,20\}$: $\{4,8,12,16,20\}$ — 5 elements. Their images must be multiples of 3 in $\{1,\ldots,20\}$: $\{3,6,9,12,15,18\}$ — 6 elements.
Assign the 5 multiples-of-4 to 5 of the 6 multiples-of-3 (injectively, since $f$ is onto): $6 \times 5 \times 4 \times 3 \times 2 = 6!/(6-5)! = 720$... but we need surjection on all 20 elements.
The remaining 15 domain elements map to the remaining 15 codomain elements (after fixing the 5 images above): $(15)!$ ways. But 1 multiple-of-3 is unused by the multiples-of-4, so it must be covered by remaining 15 elements.
Total $= 6! \times (15)! = (15)! \times 6!$
Slope angle \(\theta = \tan^{-1}\left(\dfrac{\text{rise}}{\text{run}}\right) = \tan^{-1}\left(\dfrac{18}{28}\right) = \tan^{-1}(0.643) \approx 33°\
In architectural design, comfortable stair angles range from 25° to 40°. The ideal is around 30°–35°.
This is why building codes specify maximum riser heights (typically 190 mm) and minimum tread depths (typically 250 mm) — to keep the angle in a safe, comfortable range.
For two Carnot engines in series where the work output is equal ($W_A = W_B$):
$Q_1 - Q_2 = Q_2 - Q_3$
Since $Q \propto T$ for a Carnot cycle, we can write:
$T_1 - T_2 = T_2 - T_3$
Rearranging for $T_2$:
$2T_2 = T_1 + T_3 \implies T_2 = \frac{600 + 400}{2} = 500 \text{ K}$.
- At C2: Priority order is $-OH > -COOH > -C(H)(Cl)CH_3 > -H$. The sequence is counter-clockwise, but since $H$ is on the horizontal line, it is 2S.
- At C3: Priority order is $-Cl > -CH(OH)COOH > -CH_3 > -H$. The sequence is clockwise, but with $H$ on the horizontal line, it is 3R.
The Aligarh Movement (1870s onward) was fundamentally an educational movement. Sir Syed Ahmad Khan sought to encourage Muslims of the subcontinent to pursue modern, Western-style education alongside Islamic learning. The Muhammadan Anglo-Oriental College at Aligarh (later Aligarh Muslim University) was its flagship institution.
Correct Answer: A — Education.
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