Define a variable
Pick the unknown. Write 'Let x = the number we want.' Other quantities can be expressed in terms of x. This converts a word problem into an equation.
Watch the keywords
'Sum/total/together' = +. 'Difference/less than' = −. 'Of' often = ×. 'Per' or 'for every' = ÷ or rate. 'Twice as old' = 2x. Translating accurately is half the battle.
Check the question, not the equation
Once you solve, re-read the question. Did it ask for x, or for something else (the larger of two numbers, the total cost, etc.)? Many wrong answers come from solving for the wrong thing.
Plug the answers in
If you're stuck, try each answer choice in the original problem. The right one will fit. This works especially well on age, ratio, and 'how many' problems.
Tip: revisit this strategy after every 10 missed questions on this topic.