Substitution is the moment the letter turns back into a number. Replace every copy of the letter with the value you are given, then work the arithmetic out in the usual order — brackets, then × and ÷, then + and −. The trap is an expression written the other way round, like 5 + 3n. The 5 comes first on the page but the multiplication is done first, so with n = 4 you work out 3 × 4 = 12 and then add 5. Writing the substitution out as 5 + 3 × 4 before you calculate anything makes the order obvious.
Worked example
Problem Find the value of 4y + 7 when y = 3.
Replace every y with its value, 3, and write it out: 4 × 3 + 7.
4y means 4 × y, so it becomes 4 × 3 = 12.
Then add the 7: 12 + 7 = 19, so the value is 19.
Tip Multiply before you add, even when the number to be added is written first.