38
Problem solving: multi-step
Textbook: Part 1, lesson 38, pp. 117–119
GoalSubstitute values in expressions with letters, calculate volume and solve time equations.
New words
expression with a letter · harfiy ifodasubstituting a value · qiymat berishvolume · hajmtrue statement · to‘g‘ri tenglik
Explanation
In an expression with a letter we put the given number in place of the letter and calculate. For example, if 5 notebooks cost x sum each and 2 albums cost (x + 1 500) sum each, the cost is 5 × x + 2 × (x + 1 500); for x = 4 000 that is 20 000 + 11 000 = 31 000 sum. The volume of a cuboid is V = a × b × c. A time equation is solved by the same rule for the unknown part: x = 9 h 05 min − 6 h 50 min.
Worked examples
A box with a 30 cm × 30 cm base and height 12 cm has volume V = 30 × 30 × 12 = 10 800 cm³.
6 h 50 min + x = 9 h 05 min → x = 9 h 05 min − 6 h 50 min = 2 h 15 min.
Class activity
“Measure the volume”: children measure a matchbox or a notebook and calculate its volume with the formula.
Practice
1
5 notebooks at x sum each and 2 albums at 1 500 sum more each were bought. If x = 4 000, how many sum were paid?
31000
2
A box has a 30 cm × 30 cm square base and is 12 cm high. What is its volume in cm³?
10800
3
Solve 6 h 50 min + x = 9 h 05 min.
2 h 15 min
4
Is it true that the smallest four-digit number minus the largest three-digit number equals 1?
Yes: 1 000 − 999 = 1