Connecting two equations

A lot of word problems become easy once you form two expressions and connect them into a single equation.
You write an expression for each quantity, then join them with the relation the problem describes — most often an equals sign. The one rule to respect is that both sides must be in the same unit.

Take an ages problem: Maya is $m$ years old now, and in $5$ years she will be twice as old as she is today.
One expression is her age in $5$ years, $m + 5$; the other is twice her current age, $2m$.
Both are ages in years, so you can connect them: $m + 5 = 2m$.

Take a cost and revenue problem: a stall's cost is $\$50$ plus $\$2$ per item, and it earns $\$7$ per item sold.
Cost is the expression $50 + 2n$ and revenue is $7n$, both measured in dollars.
At the break-even point they are equal, so you connect them: $7n = 50 + 2n$.

Take a time, speed and distance problem: a car travels for $t$ hours at $60$ mph, and a bus covers the same distance in $t + 1$ hours at $40$ mph.
The car's distance is $60t$ and the bus's is $40(t + 1)$, both in miles.
Because the distances are equal, you connect them: $60t = 40(t + 1)$. In every case, matching the units is what tells you the two expressions can be set equal.

Tom is now $t$ years old, and his mother is $30$. In $5$ years, his mother will be twice as old as Tom. Which equation correctly connects these?
(A) $30 + 5 = 2(t + 5)$
(B) $30 + 5 = 2t$
In $5$ years the mother is $30 + 5$ and Tom is $t + 5$, and "twice as old as Tom" means twice Tom's future age.
Choice (A) connects the mother's future age to $2(t + 5)$, both in years, so (A) is correct.

A food truck has fixed costs of $\$200$ plus $\$3$ per meal, and it sells meals at $\$8$ each. Which equation gives the break-even number of meals $m$?
(A) $8m = 200 + 3m$
(B) $8m + 200 = 3m$
Revenue is $8m$ dollars and cost is $200 + 3m$ dollars, and break-even means they are equal.
Choice (A) sets revenue equal to cost, both in dollars, so (A) is correct.

Two trains leave the same station in opposite directions, one at $50$ mph and the other at $70$ mph. Which equation gives the time $t$, in hours, when they are $360$ miles apart?
(A) $50t + 70t = 360$
(B) $50t = 70t + 360$
Each train's distance is its speed times $t$, and moving apart means the distances add to $360$ miles.
Choice (A) adds the two distances to $360$, all in miles, so (A) is correct.