An electric kettle has two coils. When one of these is switched on, thje water in the kettle boils in 6 minutes. When the other coil is switched on, the water boils in 3 minutes. If the two coils are connected in series, the time taken to boil the water in the kettle is

Explanation

(a) Let lengths of the sticks are named as `l_1 = 12.132 cm, l_2 = 12.4 cm`
Here `l_2` has one decimal place and `l_1` has to be rounded off to have only two decimal places
`l_1 + l_2 = 12.13 + 12.4 = 24.53`.
This is to be rounded off to have one decimal place only .
`:.` The total length is 24.5 cm
(b) `l_1 = 12.132, l_2 = 12.4`
`l_1 - l_2 = 12.4 - 12.132`
Here 12.4 has only decimal place and hence 12.312 should have only two decimal places.
`:. l_1 - l_2 = 12.4 - 12.13 = 0.27`
This should be rounded off to have only one decimal place
`:. l_1 - l_2 = 0.3`
Hence difference of their lenghts is 0.3 cm.