Time, calendars and reading scales

Time is not base ten

An hour has $60$ minutes, not $100$. So $2$ h $45$ min plus $30$ min is not $2$ h $75$ min — sixty of those minutes make another hour, giving $3$ h $15$ min. Every time-mistake at this stage comes from treating time like ordinary numbers.

Counting on a time line

To find how long from $09{:}40$ to $11{:}15$, count in friendly jumps: $09{:}40\to10{:}00$ is $20$ min, $10{:}00\to11{:}00$ is $1$ h, $11{:}00\to11{:}15$ is $15$ min. Total $1$ h $35$ min. Jumping to the next whole hour first turns a hard subtraction into three easy ones.

Calendars

Days of the week repeat every $7$. If the $3$rd is a Monday, so are the $10$th, $17$th and $24$th. Counting on in sevens is much safer than counting day by day.

Scales

On any dial or jug, work out the value of one small division before reading the pointer: divide the gap between two labelled numbers by the number of spaces (not the number of lines).