According to the meaning of the question, you can list:
Number of pens:
(4×6- 15)÷(4- 1)
=(24- 15)÷3
=9÷3
=3 (yuan)
So a pen, 3 yuan.
Calculation method:
decomposition method
According to the law of operation and the characteristics of numbers, the numbers in formulas are often decomposed and reorganized flexibly to achieve the purpose of rounding and simple calculation.
Break down into a product:
example
25×32× 125
= 25×(4×8)× 125
=(25×4)×(8× 125)
= 100× 1000
= 100000
This problem decomposes the factor 32 in the multiplication formula into the form of 4×8, and then multiplies 4 by 25, 8 by 125 to get the formula of multiplying integer hundred by integer thousand, which is relatively simple.
Decompose into a "sum":
It is to decompose a certain number in the operation into an integer thousand or an integer hundred, and an integer ten plus a number to simplify the calculation.
example
105×36
=( 100+5)×36
= 100×36+5×36
=3600+ 180
=3780
In this problem, the factor 105, which is close to an integer, is decomposed into the sum of 100+5, and then the calculation is simpler by multiplication and division.