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Modulo Arithmetic & Congruence · Question 19 of 97

When is the fifth service?

Question 19

MediumOriginal question

A factory runs round the clock. A machine is serviced every 30 hours, the first service at 06:00 on a Monday. The shifts run from 06:00 to 14:00, 14:00 to 22:00 and 22:00 to 06:00.

When is the fifth service?

Saturday at 06:00Correct answerYour answerSaturday at 12:00Correct answerYour answerFriday at 06:00Correct answerYour answerSunday at 06:00Correct answerYour answer

Show the answer and explanation

CorrectNot quiteAnswer A

Correct answer: A

The fifth service comes $4 \times 30 = 120$ hours after the first.
$120 = 24 \times 5$ : exactly 5 days later, at 06:00, so Saturday at 06:00.

Worth noting

Saturday at 12:00 counts 5 gaps of 30 hours; from the 1st to the 5th service there are only 4.

Concept: Counting on with a period
Typical timeTypical: 120 s
Your timeYours: —

The rule this testsThe modulo operation

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