# Time - 9th grade (14y) - math problems

- The half life

The half-life of a radioactive isotope is the time it takes for a quantity of the isotope to be reduced to half its initial mass. Starting with 145 grams of a radioactive isotope, how much will be left after 3 half-lives? - The coal

The coal stock would be enough to heat a larger room for 12 weeks, a smaller one for 18 weeks. It was heated for four weeks in both rooms, then only in a smaller one. How long was the coal stock enough? - Digging

The first worker would dig a trench in 6 hours. For the second, the same work would take 4 hours. How long did it take for the trench to be dug if they worked together? - Energy consumption

The device is connected to 230V and draws 3.5A current. Power consumption is 1932kJ. How many minutes has this device been in operation? - Two trains

The train runs at speed v1 = 72 km/h. The passenger, sitting in the train, observed that a train long l = 75m in 3 s passed on the other track in the opposite direction. Calculate the speed of this train. - Cooker

A current of 2A passes through the immersion cooker at a voltage of 230V. What work do the electric field forces in 2 minutes? - Bricklayers

The first bricklayer would build a wall in 3 days, the second in 4 days. The first bricklayer worked alone for a day, then the second bricklayer came to his aid and they finished the work together. How long did it take to build the wall? - Clock hands

Calculate the internal angles of a triangle whose vertices lie on the clock's 2, 6 and 11 hours. - Working together

Two people will do the work in 12 days. They worked together for 8 days. Then only one worked for 10 days. How many days would each of them do the work if he worked alone? - The shooter

The shooter heard the impact of the bullet on the target in one second after the shot. The bullet was moving at an average speed of 500 m/s. Calculate with speed of sound of 340 m/s. Determine the distance of the target. - Christmas holidays

How many percent take Christmas holidays lasting 18 days from a school year that lasts 303 days? Round the result to two decimal places - Hectares of forest

12 workers plant 24 ha of forest in 6 days. In how many days will 15 people and 12 people plant the same area? - Penalty daily

John was to pay CZK 1,500. If he does not pay on time, he will be charged a penalty of 0.5 per mille of the amount owed for each day of delay. He will remember in 30 days. How much is the penalty? - Corona virus

Calculate an estimate of the percentage of GDP (Gross Domestic Product) declined due to the quarantine related to the corona COVID-19 virus that lasts for 2 months, during which time the economic activity is down by 80%. - Self-oscillation period

The water in the vessel carried by the boy has a self-oscillation period of 0.8 s. What is the size of the boy's movement speed when the length of the boy's step is 60 cm? Give the result in m/s. - Runners

If John has a running speed of 3.5miles per hour and Lucy has a speed of 5 miles per hour. If John starts running at 10:00 am and Lucy starts running at 10:30 am, at what time will they meet? (as soon as possible) - Volume of wood

Every year, at the same time, an increase in the volume of wood in the forest is measured. The increase is regularly p% compared to the previous year. If in 10 years the volume of wood has increased by 10%, what is the number p? - Water current

John swims upstream. After a while, he passes the bottle, from that moment he floats for 20 minutes in the same direction. He then turns around and swims back, and from the first meeting with the bottle, he sails 2 kilometers before he reaches the bottle. - Baking muffins

Aunt Polly is baking muffins at a speed of 4 muffins per minute. Tom comes into the kitchen when Aunt Polly has 12 muffins, and begins to eat these muffins at a speed of 6 muffins per minute. When he finally leaves the kitchen, there are 4 muffins at the - Panel house

The construction company has two groups of workers. Group A has more members than B. Panel house was insulated by group A in 10 days. The same block of flats was insulated together in 6 days. How many days would group B block insulate?

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