Amount of salt = 30% of 1 liter = 0.3 liters = 300 g (since 1 liter of water is approximately 1000 g).

Average speed = Total distance / Total time = 150 km / 3 h = 50 km/h.

Let the original number be x. After a 25% decrease, it becomes 0.75x. After a 25% increase, it becomes 0.75x * 1.25 = 0.9375x, which is less than the original number.

Let the width be w. Then the length is 2w. The perimeter is 2(w + 2w) = 48, which simplifies to 6w = 48, so w = 8 cm.

Distance = Speed × Time = 60 km/h × 2.5 h = 150 km.

'Modern' is the antonym of 'ancient', referring to something contemporary.

'Natural' is the antonym of 'artificial', which refers to something made by human beings rather than occurring naturally.

'Rare' is the opposite of 'frequent', meaning not occurring often.

'Quiet' is the opposite of 'noisy'.

'Pessimistic' is the antonym of 'optimistic', which refers to a negative outlook.

'Fail' is the opposite of 'succeed'.

'Defeat' is the antonym of 'victory', indicating a loss in a competition.

'Gregarious' refers to being sociable and enjoying the company of others, so the synonym is 'sociable'.

'Vigilant' means keeping careful watch for possible danger or difficulties; hence, the synonym is 'alert'.

'Candid' means truthful and straightforward; thus, the synonym is 'honest'.

'Vigilant' means keeping careful watch for possible danger or difficulties; hence, the synonym is 'alert'.

'Vivid' means producing powerful feelings or strong, clear images.

Circumference = 2πr = 2 × (22/7) × 7 = 44 cm.

Let the number be n. Then n = 5k + 3 for some integer k. When divided by 10, n = 5k + 3 gives a remainder of 3.

Let the number be x. Then 3x - 9 = 6. Solving gives 3x = 15, so x = 5.

Time = distance / speed = 320 km / 80 km/h = 4 hours.

A triangle with sides 3 cm, 4 cm, and 5 cm satisfies the Pythagorean theorem (3² + 4² = 5²), thus it is a right-angled triangle.

'Barking up the wrong tree' means to pursue a mistaken or misguided course of action.

'On cloud nine' means to be in a state of blissful happiness.

'Under the weather' is an idiom that means someone is feeling ill or unwell.

'Under the weather' means feeling ill or unwell.

'Burn the midnight oil' means to stay up late working or studying.

'Hit the nail on the head' means to describe exactly what is causing a situation or to be very accurate in one's assessment.

'Let the cat out of the bag' means to accidentally reveal a secret.

'Spill the beans' means to reveal confidential information or secrets.